Everything worked out well, next dat I got a email no damages found.
We picked up our rental Citroen C3 on Friday just before midnight, 23:00, service was very fast and clear. Tomorrow in the early morning, 04:00, (Sixt is one of few that you can deliver the car this early) we will leave the car at the parking and just put the key in a box there. This will be completely in trust with Sixt, I hope afterwards everything will be fine. The car is in exactly the same condition as it was when we picked it up. I will update this review is everything turned out well.