Scratches in the car

Hello guys, I noticed today after the work that I had a slight scratch on the rear bumper. I know that he was not there yesterday because I washed my car and he was not there yet. At work that didn’t happen. I had a suspicion of staring at this morning had put a car behind me after work looking at the car and the car also had a scratch at the place where it might have driven me in. Have the Dam but where else could it not have happened. What would you do in my place? Is not so bad the scratch but already very annoying because I know that he was not there before and I actually wanted to sell the car. So some can think that it is an accident car. Ask for your help.