That totally sucks. It annoys me when others have so little consideration for other peoples property, but i guess thats just the way some people are. I try to minimise the issue by parking in places that limit exposure to damage but sometimes you just cant tell.
A couple of years ago it was a lovely day so decided to take my car out for a drive to visit friends. Had a lovely meal and some drinks so couldnt drive home but parked the car safely in a nice out of the way place. Next morning woke up to find someone had keyed the rear wheel arch. I just hope karma gets them in the end.
Thing to consider is this is just one of the low points. Think on the positive, Sly will sort your motor our, try and polish out the marks or get them sorted then summers here to enjoy the car. It will all be forgotten when you start her up and open the windows