I got married a month ago. We had a mail box type card box. In it, we found a lone $100 floating around in there. No note attached, nothing. None of the cards were open so it didn't fall out of anywhere. So I'm in the middle of thank you notes... but what to do about the anonymous giver??? Has anyone had this happen???
I would stress it too much. As long as all of the cards you received get thank you notes, I'd say you are ok. Perhaps the person who dropped the $100 in the box, doesn't want to be thanked and wishes to remind anonymous.
"I came here tonight because when you realize you want to spend the rest of your life with somebody, you want the rest of your life to start as soon as possible." - Billy Crystal, When Harry met Sally
I've not had it happen to me but it is actually not uncommon in our families for "anonymous" donations to be made to the happy couple. I know my parents and I both snuck extra $$ in the card box at my brother's wedding.
(We still haven't told them where the extra $$ came from, and they have brought it up a lot!)
Don't sweat the thank you card... someone wanted to give you a little surprise
"If we weren't all crazy we would go insane" ~Jimmy Buffett
Consider it an act of kindness! My daughter & son-in-law had 1 gift without a card! Unfortunately, they cannot send a thank you card to a an anonymous person! We did not find a left over card. Don't worry about it.
Something like that happened to us. We got a badass microwave, and we have no idea where it came from. when we opened our gifts, there was no card attached to it... although after seeing our video of the gift table, there WAS a card attached to it. Anyway, we took a picture of it, and emailed a thank you to everyone, and we put a thank you on our wedding website (it's below if you want to see it). I really wouldn't worry about it though.