Augment the stealth system by adding a monster AI attribute representing “hasn’t sensed you yet, or has lost sense of you”. Also allow players to take a deliberate action to conceal themselves. This would allow for players to hide in (or flee into) a corner or closet and allow a monster to pass them by, then get the jump on them afterward (perhaps in the form of a bonus for attacking an unsuspecting monster). Some monsters should ideally be able to conceal themselves as well, perhaps even by casting illusions that make a door appear as a wall or something to the hero, until discovered.