Remember they do an encore after the lights come up (usually). So because the lights are up, if you see their sound guy still at the mixing board, they are probably coming back. He might step out for a second, but every time I have seen them they have done this.
Oh, and I am missing this one because tomorrow is Meat Beat Manifesto, and after a 2am night Sunday (Comets on Fire) I can't keep it up.