On the assembly line, build sheets were used on every subassembly and there were several once all those subassemblies were combined. GM tried to remove them, but some which were hidden survive. None of the build sheets were deliberately placed in cars.
Some areas:
1. The top of the gas tank. This tends to be common on Van Nuys cars, and unfortunately, is often the only one you'll find. Even though the car has no frame, the chassis including rear suspension were coming along on a jig and there was a "body drop." There were some different gas tanks used in the late 70's but that may not be as important for the earlier years.
2. Under one or more of the seats. The seats were assembled at the factory in the "cushion room" and they traveled out and intersected with the main assembly line. I went to the Norwood reunion and got lots of clarifications on this. The build sheets were tucked into seats to make sure you matched up to the right car once that assembly line arrived at the cars. The rear seat frame is a good place to hold the sheet, and that's why it's there, but they can sometimes be under a front seat laced into the wires. These are more commonly found on Norwood cars.
3. Under the carpet. I think these are just random, like maybe if it fell off the dash, the carpet guy just covered it up.
There is the potential that they could be any other hidden place, such as inside the trunk or behind a fender, but again, none of this is deliberate.
It's possible that a build sheet in your car may be actually for a different car.