When something is hidden, you have no way of knowing whether it's there or not. For all you can tell, it may have teleported away. It is necessarily also unseen, because if you could see it, then it would no longer be hidden.
When something is unseen (but not hidden), everyone still knows exactly where it is, because invisible creatures are constantly emitting a high-pitched ping to everyone around them (unless they spend an action to suppress it, at which point you have an opposed check of Stealth v Perception).