[mac]

Many-many years ago I worked for one as a summer job, and you would be surprised how many cars they lost! I went to a garage to pick a repaired one up and they asked me when we were going to pick the other up as they had got so fed up with it they had moved it with a forklift truck. It was a new focus that had done 55 miles that someone had taken there 6 months earlier and forgotten about!
Or the other regular was driving round Stansted carpark looking for a telltail marker that it was a "insert brand here"hire car and finding out it had been there for weeks or even months with the keys sitting in it, most was a £1500 parking ticket (shortstay) was just easier to "lose" the ticket and get a £60 replacment.
As long as you have the PRI (pre rental inspection) that says you returned it they can't do anything