When the tag is moved, it automatically begins broadcasting a signal on the frequency. There are towers all over the County which listen for this frequency. When the towers go "hot", units begin heading that way to try to pick it up with their in-car equipment (the helicoptors all have this as well).
As far as I know, if you have the frequency programmed into a scanner, you can hear the signal being broadcast. It's probably nothing interesting (just like Lojack), but at least you'd know it's broadcasting. If you had RDF stuff, you could probably follow it around (but don't get involved...bad juju). When the tag gets taken, it's usually only 10 minutes or less before they're behind the suspect vehicle (if they get the signal).
BTW-RAT bait stands for Robbery Apprehension Team. I think ETS means "Electronic Theft Sensor", but I'm not sure.