I'd blame it on a stray bullet from a drive-by!
A bullet would probably have went through really, or not if it was traveling a long way.
Did you find anything on the ground next to the car when you came upon it? That could be a clue.
That sucks though, $100 isn't that bad though, honestly, for a broken window.
I'm still living with a cracked windshield because during the VERY few ice storms we have here in GA, I used a friggin hammer on my windshield to break the ice off. 1. It leaves scratches on the glass. 2. It further extended a dime crack I had there before, now it almost coveres the windshield, but only on the passenger side(yes, I'm an idiot for that!)
So, I still gotta get that fixed someday, or wait till someone hits me and blame it on them, hah.