I'm sorry, Wm, but you're wrong. Clearly the bag was in plain site. Just look at the facts! 1) Broken window; 2) backpack is gone. I mean, how much clearer do I have to be??
Hot Tub Time Machine Backpack probably broke the window and found a new owner because it was so pissed you abandoned it in the car. You should've been rocking that backpack with pride. Don't mess with magic.
It was down in the floorboard, but I will agree to take 10 percent of the blame. (Though I'm sure the thieves of the world appreciate your support.)