My understanding is they can get a current location with just a subpoena (not a warrant) already, but looking at a history of past locations to watch their movements constitutes a Fourth Amendment search requiring a warrant and probable cause.

Originally the Third-party doctrine[1] also would allow searches that were otherwise illegal, but recently exceptions have been made for GPS/location history in the courts in recent years (Carpenter and Chatrie).

[1] https://en.wikipedia.org/wiki/Third-party_doctrine