I'm really surprised by the results.
So, where do you keep your buildsheet? Isn't that kinda like a fender tag and shouldn't it go back in the car too? Much less likely someone would steal a buildsheet when it's behind the seat, but I'm guessing not many of us keep the buildsheet in the car. Why not? Why not at least a photo copy it?
My build-sheet is kept with all the other documents for my car, in a folder in acid free liner, as suggested by Barry, such as the pink slip, owners sales receipt, add when I found my car and even the name of the previous owners, and the direction to the person house I bought it from.
As Cody said, I plan on putting photo copies of the build-sheets back into the car, behind the rear seat, in the front seat back, above the glove box, but not under the carpet. When or if I ever sell my car the new owner will get all of the documentation I have.