The Fleeting Targets Exception is an exception to the exclusionary rule that applies in certain circumstances where the police attempt to stop a suspect who is in a vehicle and is quickly moving.
Under the exception, the police may make a warrantless search or seizure of a vehicle without violating the Fourth Amendment as long as the circumstances make it impracticable for the police to obtain a warrant.
The Supreme court has established that the exception applies if the police have probable cause to believe that the vehicle contains contraband or evidence of a crime, and the police have a reasonable belief that the vehicle is about to leave the jurisdiction or that the evidence would otherwise be lost.
In other words, the fleeting target exception allows police officers to make a warrantless stop if they have a reasonable suspicion that the suspect is committing a crime and that they may flee or lose evidence before obtaining a warrant. However, the exception only applies to the search of the vehicle and the things in it that are in plain view, and any further search would require a warrant.