Its hard to say if it is a guy or a girl doing it.
A guy pissed at you, and is a that big of a D**K to mess with a car, would really do something horrible. Coke on the paint, brkae fluid, etc, etc, etc.
A girl would do the little things like that, but since they are baloons and silly string it seem more party like.
Was there any partys at the complex when you were gone? I am assuming it was only your car that had this happen?
Either way, get a car cover that locks and use it when you go away for more then a few days.
~K