when I pick up PC cars at DEN airport, the contract lists the lot and space of the car I select. Which means the lot staff will either park the car in a specified location OR they note which location they've put the car into.
Now, if they screw that up, you are SOL...but if the contract matches the physical location, and that indicates it is a standard spot, the proof exists in their own records.
Interestingly, they will have cars in the regular area sometimes and other times the same cars are in the upgrade area (RR evoque, QX60s). I'll take both, but only when it isnt a $50 a day upgrade!