I think the worst thing that would happen is that you'd be asked to leave. I don't think anything worse than that could happen (though don't quote me on that).

I've done something similar one time when a minimalist surfing trip took a wrong turn. I learned that if you (a) have a car where the seat goes down flush with the trunk or hatchback (b) have a good sleeping pad (think REI) and warm sheets, then you can get a great night sleep and look pretty inconspicuous from the outside (no one can see in very easily since you're below the window line).

Also, placement within the parking lot is important, not too close, not too far away.