I had an interesting experience myself on a MUC -> FRA rental. In MUC the car was damaged ... which you find after a 5 minutes walk. So 5 minutes back to the desk so that you can walk back to the car and wait around 10 minutes for the damage guy. The damage guy tells you "oh in Frankfurt they dont care about it...." I said F%# that and spent ten minutes pointing out the most minute scratches.

Of course in frankfurt they pulled out the microscopes when they checked me in and spent ten minutes scrutinizing the car luckily they found nothing interesting.

Now for OP this is the funny part - funny from my perspective of renting in the US where you get a receipt from their handheld and email. I asked for a receipt since I wanted something saying everything is ok and no further charge. The guy told me he cannot do this I should go to the office and pointed me to it. Since I had a lot of time before my flight I said oh what the hell. And there in the garage there is an office that all the rental companies share where they give you a letter in German confirming that you returned the car... not a receipt. If you want a receipt you have to walk 15 minutes to the actual sixt desk and give them the confirmation that you returned the car.... I thought Germany was supposed to be a modern country