Mystery Thank You

There wasn't too much mail when I checked the mail box today, but one of the three envelopes inside was a small one addressed to me, in the size and shape usually of a thank-you note.
As I checked to see what it was, I noticed that the handwriting, kind of small and neat and not necessarily leading me to think it was from a person of a specific gender, only used my name and address, and did not put down a return address, making me more curious. When I opened it, I saw that it was indeed a thank-you note, red ink on white paper; but inside, there was nothing printed on it and no signature or any other kind of writing, just blank. I looked over the envelope for a while, and saw that there was only a postal stamp of "Buffalo 142," with no full zip code.
So, the mystery is this: who sent me this thank-you note, and was it an oversight that nothing was written in it, did someone put the wrong thank-you note in my envelope, or is someone commenting on something I wrote here, on our Buffaloroots website or one of the publications I freelance for by saying "thanks for nothing."