In this case a dog sniff of a temporarily disabled motor vehicle yielded discovery of heroin and cocaine within the vehicle. We must decide whether a police officer properly engaged in a community caretaking function involving the disabled automobile effected a seizure for constitutional purposes by requesting consent for a canine sniff of the exterior of that motor vehicle.
*839A judge of the Superior Court allowed the defendant’s motion to suppress the heroin and cocaine. A single justice of this court granted the Commonwealth leave to appeal from that interlocutory order. See Mass. R. Crim. P. 15 (a) (2), as appearing in 422 Mass. 1501 (1996). We reverse the Superior Court decision for the reasons that follow.
Background. We recite the essential facts as found by the motion judge, supplemented by undisputed testimony from the suppression hearing. See Commonwealth v. DePeiza, 449 Mass. 367, 368 (2007).
On June 9, 2006, at approximately 7:35 p.m., the defendant’s automobile, an Acura sedan, ran out of gasoline while traveling south on Route 140, in an area where the road was a four-lane divided highway with a posted speed limit of fifty-five miles per hour. At the same time, a State trooper on routine patrol with his drug-detection police dog was driving on the highway in a marked cruiser. The trooper noticed the defendant’s vehicle abruptly slow down, with its hazard lights flashing, and pull into the breakdown lane. The trooper activated the cruiser’s blue lights and parked directly behind the Acura. The defendant got out of his car, indicated that he needed assistance, and approached the trooper. He said: “I’m out of gas, what should I do?” The trooper stood by the defendant while the defendant used his own cellular telephone to call a ftiend to bring gasoline to him.
The trooper observed that the vehicle had heavily tinted windows. However, because the front windows were rolled down, he was able to see several bottles of water, some empty and some full, in the rear of the car. The trooper also noticed an air freshener hanging from the rear-view mirror, and smelled a strong air freshener scent coming from within the automobile.
As the two men stood outside the Acura waiting for the arrival of the person bringing gasoline, the trooper engaged the defendant in conversation. The defendant appeared nervous. He told the trooper that he had been shopping for clothing at a shopping mall, but the trooper noticed no shopping bags in the passenger compartment. The defendant also told the trooper that he was unemployed and had recently moved to New Bedford from Boston. He said that he had purchased the Acura approximately seven months earlier, and that the vehicle was registered in the name of his girl friend. The trooper commented on the *840Acura’s special that “Fish and Wildlife” environmental registration plate, suggesting that “it must be nice to support something that you believe in.” When the defendant did not respond, the trooper asked, the defendant whether his girl friend was “into fish and wildlife.” The defendant replied, “[Y]eah, I think so.” The entire conversation took place during a period of approximately twenty to thirty minutes.
At that point, the trooper asked the defendant for his driver’s license and automobile registration. The defendant produced both items from inside the Acura. The license identified the defendant, and the registration indicated that the Acura was registered to a woman in New Bedford. While the trooper conducted a computer check of the license and registration, the defendant sat in the driver’s seat of the Acura. The computer check revealed that the defendant had a valid license and no outstanding warrants, and that the vehicle had not been reported stolen.
The trooper handed the license and registration back to the defendant through the passenger side window. The trooper observed the defendant’s wallet on the dashboard, and a bulge in the right pocket of the defendant’s pants. He asked the defendant whether he had any weapons on his person. The defendant responded affirmatively, removing a small pocket knife from the pocket of his pants and handing it to the trooper. The trooper noticed that a bulge remained in the defendant’s pants pocket and inquired about it. The defendant responded that he had a “wad of cash.” The trooper did not ask the defendant to produce the remaining contents of his pocket at that time.
The trooper, who had been trained in narcotics trafficking, asked the defendant whether he would consent to the trooper’s police dog sniffing the exterior of the Acura. The defendant gave an affirmative response, stating that he “had nothing to hide.” The defendant got out of the vehicle and stood approximately thirty feet away. The trooper retrieved his dog from the cruiser. The canine performed an exterior sniff of the car. When the dog reached the driver’s side door of the Acura, it pointed its snout up toward the open driver’s side window. The trooper understood the dog’s actions to mean that the dog detected the scent of narcotics emanating from the Acura’s interior. The trooper asked the defendant whether there were at that time, or had ever been, narcotics inside the Acura. The defendant responded that he had *841friends who would borrow the vehicle on occasion and smoke marijuana in it. He told the trooper he used air freshener because he did not care for the smell of marijuana in the vehicle.
The trooper next asked the defendant whether he would consent to the dog searching the interior of the vehicle. The defendant responded that it was not a problem. The dog entered the car and indicated that it recognized the scent of narcotics under the rear passenger seat area.
After securing the dog in the cruiser, the trooper returned to the Acura, raised the rear seat, and discovered a door to a hidden compartment cut into the vehicle’s gasoline tank. Inside, the trooper found packages wrapped in cellophane containing what appeared to be heroin.1 The defendant was arrested. A search of the compartment revealed packages later determined to be of heroin and cocaine. An inventory search, conducted after the defendant was placed in custody, revealed cash in his pants pocket in the amount of $306. The defendant was indicted on charges of trafficking in 200 or more grams of heroin in violation of G. L. c. 94C, § 32E (c) (4); and trafficking in one hundred or more grams of cocaine in violation of G. L. c. 94C, § 32E (b) (3).
After an evidentiary hearing, a Superior Court judge allowed the defendant’s motion to suppress the drugs. The judge ruled that, although the trooper’s initial questioning of the defendant was a proper part of the community caretaking function, the trooper lacked any reasonable suspicion of criminal activity to justify further expanding the scope of inquiry. Once the trooper determined that the defendant needed no assistance, had no weapons, and had produced a valid license and registration, the trooper had no basis for further inquiry and his investigation should have terminated. The judge concluded in addition that the trooper’s decision to conduct the canine sniff of the exterior of the vehicle was not reasonable. Finally, the judge ruled that the Commonwealth had not met its burden of establishing that the defendant’s consent to the dog sniff was given freely and voluntarily.
Discussion. When reviewing a ruling on a motion to suppress evidence, we accept the judge’s subsidiary findings of fact absent clear error. Commonwealth v. Eckert, 431 Mass. 591, *842592-593 (2000), citing Commonwealth v. Yesilciman, 406 Mass. 736, 743 (1990). Although we accord substantial deference to the judge’s ultimate findings, we independently review the correctness of the judge’s application of constitutional principles to the facts as found. Commonwealth v. Eckert, supra at 593.
It is not contested that the encounter began as an appropriate community caretaking one. “Local police officers are charged with ‘community caretaking functions, totally divorced from the detection, investigation, or acquisition of evidence relating to the violation of a criminal statute.’ ” Commonwealth v. Evans, 436 Mass. 369, 372 (2002), quoting Cady v. Dombrowski, 413 U.S. 433, 441 (1973). In carrying out the community caretaking function, a police officer may, among other things, check on a stopped motor vehicle in the breakdown lane of a highway. See id. at 372-373. A check by a police officer on the status of a vehicle and its occupants falls within the scope of the community caretaking function when its purpose is to protect the well-being of the vehicle’s occupants and the public — and not when the purpose is the detection or investigation of possible criminal activity. See Cady v. Dombrowski, supra; Commonwealth v. Knowles, 451 Mass. 91, 95 (2008); Commonwealth v. Murdough, 428 Mass. 760, 764 (1999) (inquiry into condition of driver of vehicle stopped at highway rest area, for purpose of determining whether driver posed extreme danger to self or others, came within scope of community caretaking function). “The very limited and focused inspection of a vehicle to determine whether assistance or aid is required is a minimal intrusion on the occupant’s, or owner’s, expectation of privacy.” Commonwealth v. King, 389 Mass. 233, 242 (1983).
A noncoercive inquiry initiated for a community caretaking purpose may ripen into a seizure requiring constitutional justification. That, however, is not what happened here. While the usual indicia of a seizure is that the suspect would not reasonably believe that he is free to leave, see Commonwealth v. DePeiza, 449 Mass. 367, 369 (2007), that formulation must be adapted when a motorist is stranded and is not free to leave for reasons wholly apart from the police conduct. In a traffic stop, prolonging the stop beyond the license and registration check and the issuance of a citation may itself transform that stop into an investigative one. Here, the police officer did not prolong the stop; the *843driver had to wait for his friend to arrive with the gasoline. For a stranded driver, the inquiry is whether the driver would reasonably believe he was free to leave if the problem that caused him to be stranded were corrected.
When performing community caretaking functions involving a disabled vehicle, a police officer is justified in asking for a driver’s license and registration. Commonwealth v. Evans, supra at 374-375. Such a request is a minimal intrusion on the defendant’s rights and does not involve an improper seizure. Id. at 375. Thus, the trooper’s request to examine the defendant’s driver’s license and registration was appropriate.
Accordingly, the officer was properly present at the scene of the disabled motor vehicle while the defendant waited for his friend to arrive with the gasoline. See Cady v. Dombrowski, supra at 441, 443; Commonwealth v. Evans, supra at 372-373. Indeed, if the officer had left earlier, as dusk approached, passing cars would not have been warned of the presence of the disabled vehicle on the side of the road by the flashing blue lights of the police cruiser, and the driver would have been stranded if his friend had not come as promised.
While awaiting the arrival of the driver’s friend called to bring gasoline, the officer spoke with the driver. That conversation did not bring about a stop in the constitutional sense.2 Similarly, the officer’s various observations did not transform the encounter *844into a seizure. A police officer’s notations of potentially suspicious behavior or of objects in plain view in the vehicle do not themselves create a constitutional stop.
Nor was the defendant seized when the officer asked him whether he would consent to a canine sniff of the exterior of the vehicle. “A person is seized by the police only when, in light of all the attending circumstances, a reasonable person in that situation would not feel free to leave.” Commonwealth v. DePeiza, supra at 369. Nothing in the record supports the defendant’s claim that it was reasonable for him to conclude at that point that he was no longer free to leave and that his cooperation was no longer voluntary. No action or statement by the officer communicated that the defendant would be detained involuntarily should he choose not to cooperate.
On numerous occasions we have held, in circumstances at least as coercive as these, that a suspect would reasonably understand that he was free to leave, and that therefore the encounter with the police was not a stop. See, e.g., id. at 368-371 (shortly after midnight, officers drove by defendant, reversed direction and drove back, called from car to engage defendant in conversation, then left car to continue encounter; held defendant free to leave until officers announced intention to conduct patfrisk); Commonwealth v. Rock, 429 Mass. 609, 611-612 (1999) (after nighttime shooting, officers in vehicle followed running suspects, caught up with them, asked if they could talk “for a second,” and got out of police car and conversed with suspects; held not a stop because suspects free to leave); Commonwealth v. Stoute, 422 Mass. 782, 785, 789 (1996) (seizure occurs for purposes of art. 14 of Massachusetts Declaration of Rights when pursuit initiated with obvious intent to require suspect to submit to questioning). While here the defendant’s vehicle was immobilized and he had little choice but to remain at the scene, the breakdown of the automobile was not the result of any police behavior and did not render the defendant’s stay compulsory because of any official action. The defendant could not reasonably have believed that he would not be free to leave once his friend brought him gasoline. Therefore, he would not have believed himself in custody when the officer asked for his consent to a canine sniff of the vehicle. A seizure did not occur merely because of a noncoercive request *845for consent made during a permissible encounter between a police officer and an immobilized motorist.
We turn therefore to the question of consent. The motion judge viewed the defendant’s consent to the exterior dog sniff as the product of an unlawful detention and thus not voluntary. See Commonwealth v. Torres, 424 Mass. 153, 163 (1997). No consent was needed. In Commonwealth v. Feyenord, 445 Mass. 72, 73, 82 (2005), cert, denied, 545 U.S. 1187 (2006), we stated that the use of a drug-detecting dog to sniff the exterior of an automobile did not constitute a search under art. 14. “[T]he dog’s sniff and resulting ‘alert’ would constitute a search only if society were prepared to say that the defendant was reasonable in his subjective expectation of privacy in the odor of cocaine emanating from the car. We think that society is wholly unprepared and unwilling to take that step.” Id. at 82, quoting Commonwealth v. Feyenord, 62 Mass. App. Ct. 200, 207-208 (2004). “[T]he court correctly concludes that a dog sniff of the exterior of an automobile is likewise not a search within the meaning of art. 14 of the Massachusetts Declaration of Rights.” Id. at 88 (Greaney, J., concurring).3
Once the dog indicated that it had smelled an odor of narcotics coming from inside the vehicle, probable cause existed to search the car. The dog’s positive alert, together with the facts that the defendant was nervous; was in possession of a “wad of cash” despite being unemployed; said he had been shopping but had no shopping bags in the passenger compartment; and had no information about the “Fish and Wildlife” registration plate on the car, provided probable cause for the search. See Commonwealth v. Sinforoso, 434 Mass. 320, 324 (2001) (“Once the dog indicated the presence of narcotics in the rear of the car, [together with other facts,] the police had probable cause to search the car”). Therefore, the search of the defendant’s auto*846mobile and the subsequent seizure of the cocaine and heroin were lawful.
The order allowing the motion to suppress is reversed and the matter is remanded to the Superior Court.
So ordered.
At the same time, the trooper observed the arrival of a pickup truck driven by the person bringing gasoline to the defendant.
The essence of the dissent is that any inquiry that extends beyond the immediate concerns justifying the community caretaking function transforms the encounter into a stop. Here, according to the dissent, the officer could not permissibly question the defendant beyond ascertaining the defendant’s problem and arranging a solution (for gasoline to be brought to him). The dissent’s position is that further inquiry becomes investigative and impermissible because reasonable suspicion was lacking. The proposition is totally inconsistent with existing law on the subject. The police are entitled to interact with citizens, see Commonwealth v. Evans, 436 Mass. 369, 372 (2002), and such interaction is not considered a stop. See Commonwealth v. Thomas, 429 Mass. 403, 406 (1999) (police did not seize defendant merely by approaching him on public street and asking for his name and address); Commonwealth v. Sanchez, 403 Mass. 640, 644-645 (1988) (no seizure where police merely identified themselves, asked to speak to defendant, and requested consent to search); Commonwealth v. Gunther G., 45 Mass. App. Ct. 116, 117 (1998) (“The officer’s approach without any direction to stop, coincident with his asking first the three males, and then the defendant a second time, to talk to him, did not constitute a seizure”).
Although the discussion in the Feyenord case, see Commonwealth v. Feyenord, 445 Mass. 72, 82-83 (2005), cert, denied, 545 U.S. 1187 (2006), regarding the lack of expectation of privacy in an odor of narcotics emanating from the car is instructive, the present case is distinguishable. In Feyenord, the court concluded that, after a motor vehicle stop, detention of the defendant for a brief period in order to bring a drug-sniffing dog to the scene was permissible in light of reasonable suspicion of criminal activity. Id. In the instant case, the police did not immobilize the car, and thus the considerations underlying the Feyenord decision are not present.