Ah sorry, guess I replied too fast.
When I bought a Zotac GTX 980 a few months ago, I got an email from Nvidia with a code for The Witcher 3 which I had to redeem here:
https://redeem.geforce.com/en-usNot really sure how Nvidia got my email and how they knew I bought one of their cards. But I guess it was the online shop where I bought it that registred my card automatically or something.
Sometimes you can find the S/N on the card that differs from the one on the box.
EDIT: Btw. what do you mean by you succesfully downloaded Batman Arkham Knight last time? Did you buy a second grapchis card recently?