This has to be one of the strangest things I've seen Grim do. Fairly early in my most recent Sunset Valley game, I noticed there was a graduation going on, so I zoomed in. It was Holly Alto, Bebe Hart, and... Grim Reaper?
It was definitely Grim and not an invisible or bugged sim, because when I moused over him, his name came up. After the graduation was over, he didn't de-spawn. He just stood there reading books until late that sim night. Eventually I reset him because I wasn't sure if he was stuck or what.