A "protective sweep" is the method of searching for additional suspects or weapons after a prime suspect has already been arrested. When a lone suspect is arrested and taken into custody, the response officers or tactical team will ask the suspect, "Is there anyone else in the house?"
Regardless of the suspect's answer, the police officers or tactical unit will still "sweep" the vicinity for additional suspects who might be hiding. This is because unknown or hidden persons—whether or not they are suspects in the case—could pose a danger to the officers' safety and lives. This is the reasoning behind the police's use of "protective sweep."
However, it has become an increasing trend for police officers to use this excuse to conduct illegal searches that are not covered under the parameters of a search warrant. This is why "protective sweep" has become a source of controversy.
Despite criticisms from civil rights activists and lawyers, the legality of protective sweeps was defended by the US Supreme Court, and its justification was outlined in Maryland v. Buie (1990) 494 U.S. 325, stating,
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 immediately launched.
Recently, the California Supreme Court opined that the justifications outlined in Maryland v. Buie were not reasonable, especially if the arrest was made outside the residence. Remember, protective sweep only applies to the area in close proximity to the suspect or place of detention.
In People v. Celis (2004) 33 Cal.4th 667, police officers detained a drug trafficking suspect behind his home and then illegally entered and searched his home, citing protective sweep as justification. However, the California Supreme Court ruled that there was no immediate danger to the officers inside the home, and therefore protective sweep should not have been applicable.