Stealth mode is the state of using the hide and move silently skills in an attempt to remain undetected. These skills are opposed by each creature's spot and listen checks (respectively), with significant penalties to the opposition unless the other creature is in detect mode. It is toggled on and off with the "Stealth" option (leftmost) from the "Special Abilities" submenu (leftmost) in the radial menu (speed dial: 044). Characters in stealth mode cannot run, and move slower than a normal walk if also encumbered or in detect mode.
The most surprising mechanic of stealth mode might be that hiding does not require anything to hide behind; for example, it is possible to hide in the middle of an empty room. However, stealth mode does not make the hider disappear (unless hide in plain sight applies). That is, a creature that could see the hider at the moment of entering stealth mode continues to see the hider until something breaks line-of-sight.
When a hider enters the line of sight of another creature, opposed hide and spot rolls (repeated periodically) determine if the hider is seen. Once seen, the hider remains seen by that creature until line-of-sight is broken—additional opposed checks are not necessary. If the hider remains unseen, the opposed move silently and listen rolls may still result in the hider being heard. Unlike spot checks, a successful listen check applies only until the next check is made. So a hider that is unseen but heard one round could become undetected the next.
Stealth rolls are made every six seconds, but are checked more often than that because there are several situational modifiers to the rolls. (For these factors, see the listen and spot articles.) When a player character is doing the detecting, the opposed rolls (with current modifiers) are checked five times per second, while a detecting non-player character makes these checks every four seconds.
Stealth mode is not canceled by any conditions the character might become subject to other than death. This includes conditions that make a character merely uncontrollable (such as fear) as well as conditions that are "almost death" such as petrification. Even a dying player character (less than 1 hit point but more than -10) will remain in stealth mode.
If both skill checks (hide and move silently) are successful—and if stealth is not negated by true seeing—then the hiding character is undetected. Undetected by a player character (PC) means the hiding character is not rendered on that player's screen. Undetected by a non-player character (NPC) means the AI is not informed that the hiding character is nearby. If the hide check fails, then the hiding character is rendered on the player's screen normally, and the AI receives an OnPerception event for being seen, which is generally the main trigger for an NPC reacting to others. If the hide check succeeds but the move silently check fails, then the hiding character is rendered translucently on the player's screen, and the AI receives an OnPerception event for being heard, which is often ignored (to prevent NPCs from reacting to creatures on the other side of walls and doors). If the hiding creature is the player's own character, then skill checks do not affect the rendering; the PC is always rendered translucently to represent being in stealth mode. Associates of one's own PC are similarly rendered translucently when first entering stealth mode, but sometimes they get rendered normally after being out of visual range (such as during an area transition).
The degree of success of stealth has implications for targeting. Perhaps the most obvious is that when a creature is not rendered on a player's screen, it becomes rather difficult for that player to target that creature with attacks, spells, special abilities, etc. Less obvious is that a partially detected (heard but not seen) creature has limitations on what can target it; it can be attacked (with a penalty for not being seen), and it can be targeted by some spells cast from a quickslot, but it cannot be targeted by spells cast from the radial menu (its "Spells" submenu is disabled).
Unlike players, the AI is capable of targeting undetected creatures. While this is usually not apparent (most of the time, the AI will not take advantage of this, causing stealth mode to usually work as expected), there are some instances where an NPC may ignore another creature's stealth mode. One of the more common cases where this happens is when an event other than perception triggers an NPC's reaction. In addition, the standard AI often ignores events related to a creature disappearing, so it might continue to pursue a creature that has run around a corner to enter stealth mode, with the result being that it acts as if the now-hidden creature can still be seen.