This is when a room looks empty of anything significant, and it's not. There's a well hidden trap that will set off if the hero either tries to leave or sets off another trigger, usually by the standard look-see or an Event Flag. If the hero is lucky, it will just be a monster to fight. If the hero is unlucky, then Have a Nice Death. Common in adventure video games and Tabletop Games. Compare He Was Right There All Along, Empty Room Psych (which this trope often leads to), Suddenly Harmful Harmless Object, Teleporting Keycard Squad, Drop in Nemesis (where the trap results in instant death).