Car at the rear (door + fender) scratched, what was that? How to proceed?

Hello, this morning I don’t look at my E90 in the back, and still think: it’s scmutzig, I realize that it’s not dirt, but that there are some bad scratches that I don’t know about. So that you can take a picture, here are two photos:http://www.abload.de/thumb/img_2463_tnboud9.jpg http://www.abload.de/thumb/img_2465_tndlu46.jpg What bothers me the most is that I can’t tell exactly when and where this happened. I have the vehicle. washed a week ago, the scratches were definitely not present. Now it is that it was very cloudy with us the past days, and I also often very late in the dark or in the morning also in the dark into the vehicle. In between I was on the way several times, while shopping, on the company parking place, etc. At my home the vehicle is on the parking space, however often neighbour children play on the property, which also ride sometimes with their scooters, Trak I can’t imagine that they’re causing something like this. For me, it looks more like a strip with a fender when parked in or out. Can you conclude from the scratches on the direction in which the scratches were created? I would be interested: – do you think these are typical traces of a foreign bumper, or rather other damage? How do I get the exact cause? – would you go to the police, and show the whole thing? What do I tell them if I can’t determine exactly the place or time? – the vehicle is fully insured, such damage falls under the insurance cover, if you can’t find the perpetrator? – should I have an expert examine the damage, if so, how do I find a good one, and what does that cost about? – if I have the expert repair done, how much do you value the damage? I know, but for the first time in such a situation, I am grateful for all the tips and hints on how to behave now.