Hello @MartynHst43277 Martyn!
It seems like unsigned cards are becoming a trend these days. Funny thing, another Postcrosser had the same scenario except she didn’t back read any of her messages first and ended up everyone searching for this mystery sender. Turns out it was a blind swap. You can check it out here.
Perhaps you can share us some more info, like cancellation marks, the sender’s penmanship and other details you think it might be helpful.