Location services + police

We know that any device (like a phone) with location services turned on is generating a large pile of data about your every movement. If you don't want Google or Apple to know that, you turn location services off.

And if you're about to commit a crime and you're planning to get away with it, you leave your phone at home, or you turn location services off well in advance and keep them off so you don't create an obvious window.

These things I knew. What I hadn't previously heard of is geofencing warrants, where police can subpoena location data for everything in range of a crime scene, dig through it, and then get an arrest warrant for the owner of a specific device. Fortunately Google gave the target a heads-up; unfortunately I do not know if that is them "just being nice" (so they could decide not to) or if they have to.