Protective Sweep-A quick and limited search of premises to protect the safety of the police officers or others on the scene.
Maryland v. Buie, 494 U.S. 325 (1990)-Police entered Buie's home to serve an arrest warrant for armed robbery. Buie was arrested as he exited his basement. The police did a protective sweep of the basement for officer's safety. Evidence was found in plain view. The USSC ruled that no warrant was required, and as an incident to the arrest the officers could, as a precautionary matter and without probable cause or reasonable suspicion, look in closets and other spaces immediately adjoining the place of arrest from which an attack could be launched. Reasonable suspicion is required to sweep beyond this limited area.
USA v. Gould, (.pdf) No. 0230629cv0p (5th Cir.)(2004)-Law enforcement officers, legally in a residence, but without a search or arrest warrant, can make a "protective sweep" of the residence if they have reasonable suspicion their safety is in jeopardy.