Courts use a two-part test (fashioned by the U.S. Supreme Court) to determine whether, at the time of the search, a defendant had a legitimate expectation of privacy in the place or things searched:

• Did the person actually expect some degree of privacy?
• Is the person's expectation objectively reasonable -- that is, one that society is willing to recognize?

For example, a person who uses a public restroom expects not to be spied upon (the person has an expectation of privacy) and most people -- including judges and juries -- would consider that expectation to be reasonable (there is an objective expectation of privacy as well). Therefore, the installation of a hidden video camera by the police in a public restroom will be considered a "search" and would be subject to the Fourth Amendment's requirement of reasonableness.

On the other hand, when the police look for and find a weapon on the front seat of a car, it is not considered a search under the Fourth Amendment because it is very unlikely that the person would think that the front seat of the car is a private place (an expectation of privacy is unlikely), and even if the person did, society is not willing to extend the protections of privacy to that particular location (no objective expectation of privacy).

A good example of how this works comes from a U.S. Supreme Court case in which the court held that a bus passenger had a legitimate expectation of privacy in an opaque carry-on bag positioned in a luggage rack above the passenger's head, and that the physical probing by the police of the bag's exterior for evidence of contraband constituted a search subject to Fourth Amendment limitations. (Bond v. U.S., No. 98-9349 (April 17, 2000).)