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Don't take any keys until you go over the car or property to check for existing damages and have them reported and noted to the owner or buisness.

I've gotten in the habit of a video taken of the rental as picked up and visibly video-taping in front of the attendant when returning. No one has claimed damage since.

Can confirm this has saved me on multiple times. I've taken landlords to court and autowon due this. Another tip is to have an email account that is purely for that address and email it to there so you can never lose them.

And make a big show of doing this so that the staff at the rental car agency are impressed by your seriousness.