I had an issue returning with Hertz at LHR a few years ago.

On return the agent noticed a scratch on the underside of the front bumper. He was literally laying on the floor to see it. He told me I had to pay something like £250 as a small damage charge. I protested and he told me the other option was to refuse to pay and Hertz would be in contact for me to pay the full cost of the repair, which could be more than the £250.

I just walked out. Never heard from them again.