Basically they just left them where ever the spirit moved them to do so. It apparently wasn't a requirement to leave them at all, much less anywhere specific. The build sheet was just a part of the manufacturing process, kind of like the plastic bags over the seats and such, and they were disposable. Once the car was built the factory was done with them and they were considered just so much waste material.
Most of them were left under the rear seat cushions. Sometimes they've been found behind the upright portion of the rear seat along the metal dividing the trunk from the passenger compartment. Sometimes they've been under the front seats, sometimes on the carpet and sometimes under the springs on the front seats. Some of them got left on top the gas tank, pinched between the tank and the trunk floor. Sometimes they've been found above the headliner. I've heard of one that got scrunched behind the passenger-side kick panel. And apparently sometimes they got left under the carpet. Of all of those, under the carpet is the only place I haven't checked on mine.
I think there were a lot more that were just left on the floor of the car when it was finished, and the flunky who got the job of cleaning the car up at the dealership as part of the EXTRA CHARGE "new car prep" tossed them out along with the plastic that covered the seats in shipping. I'm pretty sure that's what happened to mine.
