What it sounds like is the person impersonating the former resident was purged from the voter rolls thanks to your action of mailing back the registration card.
It sounds like the the special sheet may have been for people who recieved a "provisional ballot", and someone showed up impersonating the former resident, forging her signature. This sounds very much like a concerted effort by one of these shady leftwing 527 groups.
2 questions:
1) Do you live in a state with these provisional ballots?
2) Did the woman at the polling place make any note about it on the sheet?
If I were you, I would follow up on this as much as possible, including reporting it to the authorities, the election board, and your state GOP.
You can vote provisionally at the polling place, but there's no way they got the card back. I only mailed it yesterday and missed the last pick-up at that. That's why I was so concerned to mention it at the polls.