https://news.ycombinator.com/item?id=17081684
The top comment expands on the situation:
"It's not just your cell carrier. Your cell phone chip manufacturer, GPS chip manufacturer, phone manufacturer and then pretty much anyone on the installed OS (android crapware) is getting a copy of your location data. Usually not in software but by contract, one gives gps data to all the others as part of the bill of materials.
This is then usually (but not always) "anonymized" by cutting it in to ~5 second chunks. It's easy to put it back together again. We can figure out everything about your day from when you wake up to where you go to when you sleep.
This data is sold to whoever wants it. Hedge funds or services who analyze it for hedge funds is the big one. It's normal to track hundreds of millions of people a day and trade stocks based on where they go. This isn't fantasy, it's what happens every day.
Almost every web/smartphone mapping company is doing it, so is almost everyone that tracks you for some service - "turn the lights on when I get home". The web mapping companies and those that provide SDKs for "free". It's a monetization model for apps which don't need location. That's why Apple is trying hard to restrict it without scaring off consumers."
Use ultrasonic inaudible sounds for tracking you
Your TV ads could play these beacons to which your phone is actively listening.
Or websites and physical places could play them.
See: https://techcrunch.com/2014/07/24/silverpush-audio-beacons/
It was a while ago now so younger HN readers might not know about it, but Phorm was an ISP level deep packet inspection product that invaded users privacy on a massive scale, all just to market stuff to them. Thankfully it's now dead and gone.