Something like this would make it pretty easy to spot one of those stingray devices, I bet. Cell towers tend to stay put, so mapping out an area would go quick. Were a new 'tower' show up in an odd spot -- there may be shenanigans at hand.
Or maybe it's simply https://en.wikipedia.org/wiki/Mobile_cell_sites
I'm pretty sure gps location is derived from signals from GPS satellites and ground stations. You can see this if you're somewhere with no cell signal but have gps, or visa versa.