As I said, no one to my knowledge has yet been prosecuted not returning a card.
My advice, if you still have an old one is to post it back to the IB. If you have lost it or whatever, fill out a stat dec and send that instead.
As for burdens of proof as to whether it was sent back, I'll leave that to the legal experts. You could however use a stat dec to say that you did post the card on a certain date if that is what you did but be very cautious indeed of falsifying documents.