The Fourth Amendment protects against unreasonable searches and seizures. The capacity to claim the protection of the Fourth Amendment depends upon whether the person has a legitimate expectation of privacy in the invaded place. If an article is already in plain view, neither its observation nor its seizure would involve any invasion of privacy. First, the police officer must lawfully make the initial intrusion or otherwise properly be in a position from which he can view the contraband. The warrantless seizure of contraband that presents itself in this manner is deemed justified by the realization that resort to a neutral magistrate under such circumstances would often be impracticable and would do little to promote the objectives of the Fourth Amendment.
Subjective intentions play no role in an ordinary, probable-cause Fourth Amendment analysis.
As a general matter, the Supreme Court has recognized that the decision to stop an automobile is reasonable where the police have probable cause to believe that a traffic violation has occurred. And we have indicated that, during a routine traffic stop, an officer is permitted to detain the individual in order to check for outstanding warrants. It remains unclear whether a canine sniff alone constitutes a detention, or whether calling in a canine unit requires reasonable suspicion. In Place , the Supreme Court concluded that the law regarding investigative detentions should apply when police seize an individual's luggage. The Court did not decide whether a canine sniff in the absence of detaining luggage constituted an investigative detention requiring reasonable suspicion.