"To get a warrant" means to obtain legal permission from a judge or a court to search someone's property, make an arrest, or conduct other investigative actions. Full definition
What should have happened is that the officer get a warrant for the stereo and its serial number, and then return to search it. (law.stackexchange.com)
As for law enforcement, sure, the law requires police officers to get a warrant before searching a phone. (fortune.com)
Technically, the raid was not to search for drugs, as authorities couldn't get a warrant to do that. (fortune.com)