I know where mine is:
He was trying to raid the kitties' litterbox, not use it. Lol! Sitting here, hearing him gently whining, I looked around twice and couldn't find him. I had the box pulled out into the living room so I could clean the cats' area, and sat down for a YT break. The other three are at my feet, littered around my PC chair. I had looked around the outside of the box for him, but not in it!