I would guess 1 of 2 things! local punks having fun at your expense! or someone that you father sold a car to that was not happy!
My sugestion, put up a survailance cammera, you can get a complete system with nightvision camera, time lapse VCR and monitor/cable for about $800.00, maybe a bit less!
I hate this shit! someone just scratched my wifes new BMW, it's probably better I don't catch them! lol
I would go away a long time!