Something weird happened this morning...

I was getting ready to leave for work when I heard something come through the letterbox, I assumed it was the postman (posting letters through, not actually coming through the letterbox himself, although that would be a better story).
When I got to the bottom of the stairs the only thing on the doormat was a plain, blank piece of white A5 paper, nothing else! Someone posted a blank piece of paper through my door and the fact that I'll probably never know why is driving me mad!