The Katz Test:

A seminal test for the use of surveillance equipment was the United States Supreme Court case of Katz v. United States, 389 U.S. 347 (1967).

In Katz, government officials intercepted Mr. Katz’s telephone conversations by placing a listening device on the outside of a public telephone booth.

The Supreme Court, in declaring that the surveillance was a “search” in violation of the Fourth Amendment, held that the Fourth Amendment was designed to protect a person’s reasonable expectations of privacy.

Since Katz¸ the term “search” is triggered whenever there is a state intrusion in any way upon an individual's protected interest in privacy. There is a “seizure” whenever the state intrudes in any way on a protectible individual's interest in property or security.

Application: Despite the above language, the courts have required that people be extremely protective of their claimed privacy interests and that a person must manifest a subjective interest in privacy for the activity in question.

For example:

If in the Katz case Mr. Katz were shouting or speaking so loud by that someone walking by could hear his conversation, the Court would likely have found that Mr. Katz had no reasonable expectation of privacy in that particular conversation. 

Also based on the above rationale, the courts have held that a person generally has limited expectations of privacy in public places and therefore upheld video surveillance in public places.

The U.S. Supreme Court has also found that aerial surveillance of a person’s property, without probable cause or a search warrant, was reasonable and not prohibited by the Fourth Amendment because members of the public could also fly over the property and observe the marijuana growing in the defendant’s back yard.

Similarly, the Courts have found that there is no reasonable expectation of privacy in trash left for pick-up by the local garbage service since, once it is left for pick-up, it is accessible to the public at-large.

As can be expected, however, the U.S. Supreme Court and the lower courts have held that a person has a reasonable expectation of privacy in his or her own home; therefore, any surveillance must generally be supported by probable cause and a search warrant. 

Hotel Rooms - Does a person have a reasonable expectation of privacy in his or her belongings in a hotel room given that hotel staff routinely enter the room to provide cleaning and maid service?

The U.S. Supreme Court, in Stoner v. California, 376 U.S. 483 (1964), held that despite the access implicitly authorized to hotel employees, this implied grant of public access did not apply to the police.

The Homeless - What about the reasonable expectations of privacy of the homeless population which essentially reside on public streets? 

The results are mixed, with at least some state courts holding that a homeless person has a reasonable expectation of privacy in his or her belongings (duffel bag and cardboard box) that was kept on public property.

Waiver - Can a person, through his or her actions, waive their reasonable expectation of privacy in their own home? 

Apparently, yes. The court held that once the drug defendant invited confidential informant into his home, he forfeited his privacy interest in those activities that were exposed to the informant, and thus, the informant's secret video recording of drug a transaction in the defendant's residence did not violate the defendant's Fourth Amendment rights.