It is actually quite easy, from a technological standpoint, to pull off this sort of feat — particularly if the phone provider is involved. At the most basic level, when a cellphone is turned on and starts looking for a signal from a relay tower, it is announcing its location to the provider that controls the nearest tower. It is not difficult for that provider to then put together a list of cellphones in that area at a given time.
Location en context. Ik hoor het de marketeer in mij zomaar zeggen. Dan dit.