They're odd. If they were something to do with store returns, then why not put them on the box so they were obvious? Quality control makes no sense for the same reason. I doubt they're a more obscure version of the "packer" slip, either.
Thinking more about store returns, however, a couple of thoughts:
1) Maybe when an item had been opened and returned to store the staff had to take the thing out of the packaging to check it was all there. By making them put a small sticker on it this demonstrated to management that it had been opened and checked: if the staff member had to go to the effort of putting a sticker on the vehicle it's no more bother to pull it out of the box to do so. Maybe this was designed to stop the staff just accepting properly broken items back into stock?
2) If returns then maybe they were discounted in store as such. To stop customers then bringing them back and asking for a refund at a full price, prior to issuing a refund the staff would remove them from the packaging and check them over for the presence of one of these sticker. Seems like a hassle again as why not mark the box in some minor, unobtrusive way to indicate the same thing?
3) Again, if store returns maybe these indicated that the electronics had been tested by the staff upon the return to the shop and indicated that the electronics worked/it was all there and could be sold again as a return for a small discount as opposed a damaged/incomplete item which had to go back to the supplier.
Whatever they are, I doubt they're anything to do with the factory and more likely something to do with the store itself. A mystery for ever more, not doubt.