Damage to the friendly – who has a duty of proof?

Moin boys and girls, today my little one was at the 90th inspection. I brought oil and filter again, everything was good. I was always very satisfied with the AH so far. When leaving, however, I notice that the car in the height of the door handle has now about 2cm behind the handle a dent. No paint damage but only a dent. I washed the car a few days ago, the dent was not there yet. Besides, I am quite sure (estimated) 99%), that the dent was not in it when it was given away. The dent is already noticeable, especially because one looks at the area of the handle. If it were in another place, I could not be so sure. Statements of the workshop staff about the incident: Is not at the “outer” place of the door, therefore it could not have happened by “somewhere against”. Bullis or similar are not repaired in the Audi car dealership, therefore it could not have been another customer. The car in the workshop is all sharp-edged and blue, so these clear traces would leave behind. I should come back tomorrow, maybe the employee who took the car noticed the integrity of the door at the time and can now confirm that the dent is “new”. Ha ha. I’m pretty angry about it and don’t know how to react appropriately. Unfortunately, I had already paid the bill in full before… Tips? Suggestions ? Does anyone happen to know what the risk is in this case? Should the dealer make a take-off at handover, as with a rental car e.g.? da_baitsnatcha