In a 1969 decision, in
Chimel v. California, the Court ruled that if police arrest an individual, they may without a search warrant search the body of the person and the area into which he might reach, perhaps to destroy evidence or to seize a weapon. That, it said, is to protect the evidence or the officers. That is the origin of the notion that a police search incident to arrest can go ahead without a warrant.
From time to time, the Court has explored variations on the
Chimel theme, about police searches of various items individuals had close at hand when arrested, and the Justices are now prepared to look into the seizure of cellphones incident to arrest. Lower courts are in dispute on whether the Fourth Amendment allows them to search the digital contents of such a phone, without first getting a warrant.
Argument preview: Police and cellphone privacy : SCOTUSblog