The U.S. Supreme Court has reaffirmed that police need to obtain a warrant to search the premises around someone's home, even if they think they have spotted stolen property there. They aren't allowed to take a quick peek and initiate a search based on what they see.
The U.S. Supreme Court has just heard an appeal on whether police can collect cellphone location data without a warrant and still have it be admissible against criminal defendants. The case pits Americans' privacy rights against the government's interest in easy access to personal data that can solve crimes.
In order to be legal, a search warrant must be based on probable cause. In other words, the officers seeking the warrant need to document a good reason to believe that evidence of criminal activity is likely to be found in a particular location.