My Nuvi manual even stated to wipe the ring off the windshield.
I have my window broken out and Nuvi stolen on Thanksgiving day a couple of years back. Really sucks, I'll tell you. In my case, I left the Nuvi in plain view while I went after a cache in a very upscale neighborhood, thinking that it was a safe area. But in hindsight, that it was an upscale neighborhood was probably why the theives were cruising that area.
My nuvi was stolen out of our car last Christmas Eve. I think the suction cup marks on the windshield was the telltale sign. I plan to keep a box of wet wipes in the car from now on to address that problem. However, it was only while I was learning the features of the replacement nuvi that I discovered geocaching! How bizarre! If my 1st gps hadn't been stolen, I would still be clueless about this! But the addiction has already set in!