For what it's worth.

Returned car in PHX. They noticed damage. It could have been there when I drove away with the car. It definitely didn't happen when I was in/near the car, but it could have happened at some other time during my rental.

I had my regular State Farm insurance, plus my credit card insurance (MC). $500 deductible on my collision. Damage estimate was about $625. I got National to lower the bill to $500 and got MC to pay the whole thing. No claim against State Farm.

How you handle this really depends on your insurance. I didn't make a huge stink about the possibility that the car was damaged when I drove it away. Why? I suspected that National wouldn't let it go with just my word that it didn't happen when I had the car. When I spoke to the National adjuster he was quite unfriendly about the whole thing. I figured that if I told MC insurance that it must have happened before I rented the car (something I didn't know to be true), they wouldn't have paid anything. So I'd be left to duke it out with National without any help or coverage (unless I wanted to sue MC or make a claim against my State Farm policy). After all, there's nothing to stop National and MC from taking inconsistent positions.

Having considered all of that, I simply submitted the claim to MC telling them that I had no idea how the damage occurred and that it even conceivably could have occurred before I rented the car (completely true).

My only point is that sometimes fighting over all of this doesn't actually help you. In my case, my only out of pocket costs were a couple of stamps on some letters. I still don't know for sure if the car was damages when I had it, but it really doesn't matter.

Hope that helps. I must also say that I'm think about getting a Diner's Club card just for rental cars given that their CDW is primary.