I had a very similar experience with Europcar in the UK. I received a letter two weeks after I returned my car saying they were going to charge me £115 for an alloy wheel scratch. When I asked for a photo so that I could compare it with my own taken on return, they rapidly withdrew the claim. A few days later the UK press said that Europcar were being investigated for inflated damage claims.
The major rental companies (or their franchisees) seem to have the morals of the gutter when it comes to dealing fairly with customers.