The local PD in my area has/had the laptops in their vehicles set to ad-hoc mode, and each broadcast static MAC addresses in the open, and could simply be looked up on the Wigle database. At about 100-yards, you could pick up the broadcast on any phone, and it would be trivial for someone to deduce that you could setup an active monitor w/ alerts for when these specific MAC addresses were present in a designated area, let alone what a distributed monitoring/alert effort would be capable of...