RFID's are radio frequency identification "D's" (dont recall what the d stands for. It may be "detextor".
Heres a little article on the BBC bdout RFID micrchips. A "milk shake delivery" is a suitably viscous liquid in which the micro RFID's can be swallowed (or else inserted into ones lungs via an atomizer This works great with coke heads). The RFIDs can be detected by a tuned radio frequency detector (basically a radio with an energetic antenna).
You can fly ovr an area and, based upon the detetor, you can pick up the miscreants signal