You’ve forgotten something.
It’s an especially great antidote to the doorway effect, where you forget something when you walk through a doorway.
I’ve also used mental baskets for that. I’d put what I’m thinking of in a little basket in my mind before I walk through a doorway, carry it through the threshhold to the other side. Then, when I need to recall it, I open the basket and there it is.
[it’s kind of a Roman Room thing but not quite as elaborate as those]