Fraid the others are correct. Was advised when I rented a car last year to check it thoroughly before leaving the rental carpark. Even tho it was dark I managed to spot a small scrape on the wingmirror and a stain on one of the seats in addition to the couple of other marks on the car which I got the rep to agree and mark on the car's condition record.
When I left it back, I thought they'd come and do another inspection, but the car was a bit dirty and it was raining, so any inspection would have been cursory at that time. I would bet my bottom dollar they had a good look over it during the cleaning process and they had my credit card number should they need to charge me for anything they felt was wrong.
They'll find it so I'd be up front and see if the rep will be sympathetic?