This type of theft has been happening around here for a while now. Police advise that you remove the mounting device and also clean off the ring that the suction cup leaves on the glass -- thieves see the ring and that's enough for them to break into the car.

After stealing the gps, I think they usually reset them to clear out all the data, so it can't be traced back to its owner. Then they then sell them.

A friction dashboard mount is a good alternative, because it stays put while you're driving but is easy to remove from the car when you're not in it.