It depends on how they are when they leave me. I'll look for them and initiate the conversation with them, if they leave me in a peaceful, uncomplicated state with no problems keeping us from communicating to one another. But if they actually did vanish, I would try to get in touch with the other acquaintances, who could be aware of their whereabouts. When I am at my wit's end, I might send them a menfess so they can see that I'm thinking about them and miss them so much.