It sucks that the burden of identifying, reporting, and proving, is all on you. An unethical car rental location can get paid for a scratch each time the car is rented (unless the person renting reports it properly before the rental).
Something similar happened to me a couple of weeks ago. After returning from a trip out west, I received a voicemail from the car rental company saying they wanted to discuss “damages caused while you were renting” etc. followed by an Email stating I have 10 days to respond. Finally I called back and spoke to the rep. who was handling my case file and explained that “I have no idea what they want, I wasn’t in any accident and there was no damage to the car” the rep said “Ok, If there is anything else, we will reach out to you” not sure what to make of it. Sounds to me like a racket.