I have a tracker on my bike that i built my self, not bullet proof, but to find its exact location all i need to do is send my bike a txt message and I will get
a) a reply SMS with lat/lon and how long ago (in seconds) it last changed
b) will force an upload to my server which stores the location every 5mins (when ingnition is on) anyways, its just a way of forcing an update rather than waiting 5 mins.
All hidden where no one would think to look - only issue is, because is home made, it doesnt do anything (good or bad) for the insurance. But I didnt build it as an anti-theft device.
If anyone is interested in this I can come up with a price and let you know (off top of my head tracker would be < £150, and then all you need is a mobile SIM, any kind, i have a o2 prepay with unlimited txt bolt on)