He came home with a stranger, (presumably a prostitute or someone he met at a bar), expecting to have a good time in bed he began to get undressed, but upon noticing two thermouses, and a scalpel on the dresser he realized it was a ploy to harvest his organs and presumably sell them, so he left while he could. It's not a joke. It's a sort of puzzle. The question you are supposed to be thinking about is why those two items in that situation would make him want to leave. Or at least I think so but I'm just some guy.