Law enforcement uses location data for other kinds of investigations as well. Cops need to get a warrant to track a specific individual’s mobile phone. Doing it the other way around, though—picking a location, and then looking at who was there—has fewer restrictions.
Late last year, for example, federal investigators trying to solve an arson case in Wisconsin gathered location-history data from 1,500 devices that passed through the area over a nine-hour window. A similar reverse location warrant used in Florida led police mistakenly to identify a man who happened to be riding his bike near the scene of a burglary as the top suspect.
Other private entities also use geofencing tools to zero in on individuals. Ambulance-chaser law firms, for example, have advertised to people who have visited hospital emergency rooms.
While phone owners may understand that their devices collect location data for certain uses, consumers often have no idea how broadly sold and traded that information then is. That’s according to Keith Chen, a behavioral economics professor at University of California, Los Angeles, who spoke to the WSJ. “To the degree that this becomes very common, I do worry that it starts to put a chill on people’s willingness to peaceably assemble,” Chen told the Journal.