At my mom's house there's a full attic. In the attic corners, there are little rooms behind them, with the walls built out. There is access to them only through a narrow, tiny little crawlspace that goes out at the sides of the house.
Twice in our lives, I and my brother, and the same day, we crawled through there and went into one of the little rooms. I remember it clearly but forgot how long ago it was. We left a note back there on the wall.
Today, in doing some work of insulation, workmen went through the wall. I had told my Mom of the note some time back and she asked them to look for it. They said they didn't see any note. But later she went up, and of course there was the note right there on the wall.
It was in February 1978, a long time back. She's very sentimental and appreciative of stuff like this, so she left the note, except to take a picture of it. I haven't seen the picture yet. And she left a note of her own!