Usually snatch & grab is what it is.......if it is in plain veiw they will, if not, they usually dont have the time to scuffle about in the car.
Put the unit in one of the trunks, once you're in the habit, it's not that much of an inconvenience really. (just be sure no-one is watching too close when you do it!)
I bought the Mio Digi walker (great unit), and because it is a phone/GPS/PDA I gotta take it with me anyways. Maybe take a look at that unit instead??? That will solve that dilemma
Cheers