Specially we can find scratches in our cars from our own house parking during night time.. n' these kinda jobs are doing by non other than the silly guys who r thinking themselves a local nationality - acting balls.
It was happened same to our colleague once. He requested police dept. to investigate and it was gone nicely. All so-called heroes knell down and confessed their heroism.
Suggest you to contact your local police station and seek their assistance.
At least you would have save in future from such circumstances.