We left our Malibu at Offshore in their covered storage for a week two seasons ago. The only problem that we had was that we came in after hours, and they didn't pull the boat out for us. Which worked out ok, I just had to drive around and find it. But that also tells you how secure the lot is.
In regard to the parking lot, we often leave a boat in the parking lot overnight the night we get there. So far, we have not had a problem. Knock on wood.