BB gun is common for popping off windows because it is nearly silent and effective. Park your car in the garage or keep an eye on it, if they didn't steal anything they may be hoping you just see it as random occurance and will be coming back shortly to take the car since your alarm will be inneffective with a busted window and most likely unarmed.

I used to do this for a living so trust me when I say to keep a close eye on it and get the window fixed ASAP.