Your phones track you regardless of the GPS being on or off (that helps but isn't necessary). Your phone attempts to ping 2-3 different towers every few seconds, using those pings they can locate you in 3d space down to the meter.
Every single "phone ping" to a tower is saved along with the simcard information, allowing backwards tracing of your phones movement for the last ~15 years, with about 1m accuracy. The accuracy is less if you are not in a densely populated cell tower area. But even with 2 towers they can get you down to the meter in 2d space (x/y but not Z). You'll notice even small towns usually have a minimum of 2 towers allowing perfect tracking of cell phones.
There is a common sense reason why the phones have to ping towers, after all who wants their signal to drop or not switch over when moving around. However not many people realize that it can locate them so effectively and that the information has been saved for the last 15+ years.
