The homeless people/thieves here in San Francisco break off the ceramic part of a motorcycle spark plug (usually exposed on non-faring bikes). When they throw it at a car window it shatters without any sounds. Then they go in and steal the belonging inside. It's happened to me before, and my BMW (at the time) was parked across the street from the restaurant we were eating at. I saw a bum loitering around my car, but never suspected/saw him actually break the window (he did it so inconspicuously).

I take it that is how thieves do it there as well.