“Where does a thought go when forgotten?”
Hopefully you get it out of your head before that happens. Tell someone, write it down, draw it, make music with it. Something.
If you’re lucky, it’s compressed and retrievable with the right triggers. (a smell, a room you enter where you last had the thought).
If not, it might be recycled. But I think an echo of it remains even then but has to be reconstructed.