I had a bottle somewhere to mess with my sister. I teased her a long time ago and she figured out it wasn't real. Then she visited and I made sure the bottle was obviously visible in the garage
@Undo this is one of the few bits I actually know the Proper Theory (tm) of - ideally, you get a good idea of the number of vehicles expected, figure out what spaces you're gonna use, lay them out as close to venue as possible, then fill that from back to front
Think about whether traffic inside the lot will be one-way or two-way. One way will likely work fine while filling (and give you better density), but it'll be a mess when people are trying to leave