Some punk kid broke it. While I was home, nonetheless. I didn't even hear it happen. I didn't notice until I went to leave with a friend and he spotted it as we drove past my car.
After asking some neighbors, some of them saw some shady looking kids hanging around the block about the same time. I don't know if they were planning on trying to steal it and got scared off, or if they were just trying to break something. Either way, I installed an alarm after fixing the window. Won't keep someone with a brain from stealing it, but it'll scare away the idiots at least.