After my car was broken into a few years ago, and my SatNav stolen, the police told me that thieves look for the marks left by the SatNav holder on the inside of the windscreen. But it's no good cleaning the marks off, because they also look for a clean area free of marks. They recommended always cleaning the whole inside of the windscreen after using a SatNav.
Also, the majority of people keep their SatNav in the car, usually in the glovebox. The thieves know this, so not having it on display is no deterrent. Keep it in the boot or not in the car at all.