GPS receivers have varying degrees of usefulness. On the low end models, one can simply cover the receiver antenna (or simply flip it upside down) and the unit becomes non-functional. Higher end models have better antennas which make it more effective in acquiring signal.
Which leads me to the next point, GPS receivers only receive signals broadcasted by the GPS satellites. They don’t actually emit any signals of their own thus, Joe Smith can use a GPS receiver and the Man can’t track him down.
Now with a GPS tracker (something that broadcasts it’s location after receiving GPS satellite data) is different in another level. Due to the fact that the unit itself is broadcasting something. At least, that’s my understanding of it. That’s why security devices like LoJack actually work.
The real question is whether they’ve gotten GPS trackers small yet powerful enough to be detected with out the need for extensive “listening towers” (or receiving towers).
Now granted a RF tracker could probably be used with great effeciency as two (possibly three) towers used to narrow the search area. I vaguely recall a website that actually looked into stuff like this. Except I believe they were using cellphones. Unfortunately (or fortunately) that website has been down for quite some time.
Apparently their project on creating a focused EMP generator sparked some security concerns in a post 9/11 world.

GPS receivers won’t work if they can’t see satellites, and they need a straight path to the satellites because the time between the sending and the reception of the signal is critical. It’s more likely that they just put a simple radio transmitter in the bag and followed the robber by triangulation. Cops have been doing this sort of thing for 50 years or more.