I agree. The tag was probably removed years ago during a repaint or accident repairs and they never bothered to replace it. But if the prior owner really did keep it on purpose I would wonder why.
If your car is original"ish" they might be worth a couple hundred dollars, or at least thats what I might consider paying if they were greedy about it, and maybe it would motivate them to find it. If your car isnt original or you dont ever intend to return it to more or less original condition then it might not be worth much. Still, they are nice to have.
Now, if it were a real v-code or AAR or very rare car it could add much much more value than that.