A GPS receiver doesn't broadcast, and cellular triangulation is only possible if you control the cellular infrastructure (meaning either you are a government-level adversary or you have a set of Stingray-type devices). That's a fairly high bar.

You could track people around a fairly good area, like a shopping mall, using off-the-shelf, inexpensive Bluetooth hardware and just keeping track of where various IDs turn up. Indoors, that's probably much more accurate tracking than you could perform using cellular triangulation. It's more like the kind of tracking you do using RFID tokens in a warehouse.

I don't know if it's a problem per se, but it's something people should be aware of; many devices are constantly broadcasting a unique identifier almost all the time unless they take steps to turn it off.