My scientist is just going about his day when who should stroll in but Grim. He goes over to one of my coworkers and uses the "slap 'em silly" interaction, then, after standing there for a while, leaves. So I'm chuckling over this, but sorry he left the lot before I had a chance to talk to him, and then he comes back and does the exact same thing, but with a different coworker. The whole thing repeated three times before the work day was over.
I've seen Grim show up in some odd places and just hang out, but the obsession with slapping is new to me.