It's crazy how much just having your location tracked 24/7 shows about you. It can give a pretty good idea of if you're in a relationship and sexually active (where you spend your nights and when/how often), if your parents are dead and if you're married or divorced (where you go for holidays and when you stop going there), what your life expectancy is (your zip code), if you have children (when and how often you visit schools, day care centers and play grounds/chucky cheese), what you do for a living (harder now that more folks work from home), how healthy you are (time spent at doctors/hospitals/fast food restaurants/gyms), etc.

Tracking one person's location history is invasive enough, but if they're also tracking the people around you it gets a whole lot easier. Phones spend time talking to and tracking other phones around them (even when those devices are offline or have location services disabled) along with being tracked by Bluetooth beacons and collecting info about nearby wifi connections.