I don't have a d600, but I'm pretty sure multiple exposure can be a 1 time event, so if you accidentally set it, all evidence would be gone after the shot and it would revert to 'off'. On my d7000, if I wander into that menu, look at it and hit 'done' rather than backing out, it will set it for on. So if you were even looking at that menu item before the shot, consider the possibility that you accidentally turned it on for the one set. I don't think there is any indication in the resulting image file that it was a double exposure.