Stealth is the measure of how concealed the player is from an enemy NPC's attention until discovered.

Mechanic

The player is detected by NPCs through two methods: sight and sound. The player can remain undetected from enemies by hiding in dark areas, hiding in foliage, keeping their distance, using silenced weapons, and remaining silent while moving by walking or crouching, though the latter is more effective.

Generally, a good start at remaining undetected is to keep away from lights and move slowly through the environment as footsteps and the rustling of foliage can alert enemies. Stealth only works however, if the player is still undetected. Getting spotted by an enemy automatically voids any current stealth bonus one has on them.

Mutants are also more alert than stalkers due to their somewhat enhanced senses and so are harder to sneak up on, while it is outright impossible to sneak up on some at all.

Depiction in games

Shadow of Chernobyl

In Shadow of Chernobyl, stealth is monitored much more in-depth than in the other games. Both sound and sight meters can be monitored in the bottom-right bars on the HUD. As both bars increase, the less stealthy a player is.

Clear Sky

While the mechanic still exists in Clear Sky, there is no meter to determine how stealthy the player is.

Call of Pripyat

In Call of Pripyat, stealth returns, albeit is more simplified. The sight and sound meters are now located around the radar on the HUD. The green bar is sight and the blue bar is sound.