Talk to HR about possible harassment/ mind games etc. Or, suck his d*ck.

Kill him with it

I'd point it out to him before he has the time to walk away. Hopefully he'll learn but by the sounds of things he wont, so then I'd make a joke about how he's not allowed to come to his desk with things he's going to forget. With that it should make him feel more awkward than you should bringing it up.

If it leaves a ring I think the law is that you have to yell, "WHO'S CUP IS THIS? IN THIS HOUSE WE USE COASTERS, WE WASH OUR DISHES, AND WE PUT THEM AWAY."

throw it in the garbage, everyone wins🙂

Placed the cup back on his desk

Tell him he forgot it and leave it somewhere else, like the break room or on his desk.

I think it's OK to politely call/email to remind him he left it there and it's getting dumped out in the sink/trash if he doesn't come get it...or you can rub his nose in it like a puppy who wet the carpet 😛

I'm clumsy.. Ya know?... Probably it would smash against the floor bc oopsies, accidents happen.

Hide the cup and pretend you have no idea it’s whereabouts when asked.