I have used Sixt in UK, Germany, France, Spain, Mauritius, Argentina and Italy many many times and have never experienced any issues like these. I have stated this before but if you don't take photos of all damage and panels at the start of the rental and fully inspect the vehicle you leave yourself open to this kind of thing - and that is regardless of which company you use. Only two weeks ago the BMW 7er I picked up in TLS had serious damage to one alloy which was unrecorded. I pointed it out to the staff before I left the station and it was added to the vehicle damage record.

What people must realise is that staff don't always notice new damage and so it is really important that you do your own thorough inspection too.