Yep...... a pre-paid throw-away!
I hate to say it folks, but... LE has already been planting GPS trackers for a while now.
Both State & Federal agencies often employ this method without a warrant in order to enhance their ability to tail and surveil suspected
wildlife violaters, drug trafficers, car & trailer theives, etc, and the Feds haven't needed a warrant to access private property for intel since
the enactment of The Patriot Act. They just keep these dirty little details carefully covered up and off the books until they get the evidence
needed to obtain the proper warrants.
The good news is that unless there is reasonable probable cause to suspect you, they're not likely to be wasting their time.