I think I'm leaning toward the .177 caliber BB or pellet theory as well. Depending upon distance and velocity, BBs tend to bounce off targets and pellets tend to penetrate targets. Not that it matters, either way, your window is broken. Sad if it was a mischievous act.
When replacing it, you might be able to reveal further evidence of the cause.
Perhaps a slightly related story: Many years ago (early 80's) my mom was driving home from work and she heard something hit the roof as she was approaching a pedestrian overpass on the highway. My dad and I went out to look at the car. Right above the driver's side of the windshield about 6" back on the roof, there was a long, deep gouge in the roof. My dad grabbed a .22 bullet and it fit perfectly in the groove. Split second timing difference and it probably would've hit the windshield and my mom, probably causing a fatal accident at highway speeds. As we looked into it, apparently there were numerous reports of people throwing rocks and bricks off the bridge to hit the cars below as well. They put a "fence cage" all the way around the walkway to prevent objects from being thrown over, but the incidents continued. A few short years later, the walkway was taken down.
Just because we call it "civilization" doesn't mean all of the inhabitants are civilized.
Good Luck,
~Rick
2005 Georgie Boy Cruise Master 3625 DS on a Workhorse W-22
Rick, Gail, 1 girl (27-Angel since 2008), 1 girl (22), 2 boys (23 & 20).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.