GPS Navigation Devices/Am I Missing Something Here?

I just got the TomTom GO 720 for Christmas, and I think it's terrific. I do worry that my ability to feel my way from Point A to Point B will atrophy, just as I no longer remember phone numbers (they're recorded digitally in contacts), email addresses (they auto-complete), or anything else (I send emails to myself). But the point of this sort of technology is to free up my mind for the sort of exercises we do here: philosophizing, argument, irrational rants — the sort of thing computers can't do for us.

But I'm afield a bit of the reason for my post, which is this: I've heard from more than one person that I need to be careful about leaving the TomTom in the car. They're hot properties around Boston, and there have been a lot of broken-window thefts of portable nav devices. This got me thinking — shouldn't GPS nav devices be the one thing a person can't steal? When it's powered up and running, at any given time 5 or more satellites know exactly where my TomTom is. So if someone smashes out the window of my Prius and makes off with it, shouldn't somebody — TomTom seems a good candidate — be able to find it for me, the minute the thief turns it on and receives a signal?

There must be some complication here that my simple mind hasn't grasped. Or maybe theft control is one of the hundred or more ancillary services TomTom and Garmin supply at additional cost. Is it? Shouldn't it be?

Hm.