SIXT
Just to add to the body of knowledge; my son experienced this scam from SIXT at Heathrow; ie a letter sent 3 weeks after the car was returned with an unspecified claim for "damage" found "on return" of the car.
There was no damage whatsoever.
It so happens that said son is a partner in one of London's major law firms, and dealt with this false claim with a few well-chosen words.
So that's another hire-car company to avoid. Problem is that they all do this! I, not being a partner in a law firm, get a signature on the hire form from the firm's representative, name printed out etc, date and time, that the car has been inspected with no damage found. You have to plan ahead to do that and never, ever use the dreaded key-drop as they hope you will, but it's worth it.