There was a not-so-recent update that prevents the searching of winners if they did not provide a giveaway feedback yet, so what that means is if this person won the giveaway but did not mark it as received or not received, then at the winner list they will show up as anonymous but cannot be searched out.

Actually, I can't check one since I'm not member of the required groups =P
I was just pointing out a possibility, glad to know this wasn't the case :)

EDIT: Just saw PlyStar's comment.. don't know what to think now =P