As pointed out in the Monsters article, creatures are not aware of your presence until you fall within their "sensor range". Some creatures rely on direct sight contact (mostly), others have keen hearing/smell/electronic sensors. Certain predators, such as Zuul pups, will detect you 2 screens away simply because you are bleeding (Wounded). Some creatures have an area of vision that differ wildly from your character's, for better or for worse.
Stealth is not always possible, as certain creatures like Stranglers or Cyberjaeger Bears seem to be able to detect you from a distance without any cues. Murky details also makes it difficult to consistently apply this concept. However, you still want to look for an opportunity to approach a new floor "silently" when facing low resistance in the entry room (see Noise Levels).
There are many ways to alert creatures on the current floor to your presence, and most creatures emit a distinct sound once they detect you. Until that happens, however, you can sneak around certain old robots or distracted/crazed humanoids and deliver the first hit on your own terms, or scout around without being attacked. A floor entry room that is "walled off" is also ideal for a stealthy approach.
In general, logic can be applied when trying to anticipate the level of noise an action will create. Closing doors within a room limits outgoing noise range as well as preventing line of sight contact. The Stealth Armor reduces noise and possibly other "sensor range" factors.
Since noise is usually what will give away your presence in The Pit, here is a rough estimate of the mechanic behind it:
- Near silent: Caused by moving, using your Fists (or TK Fist), firing silenced weapons and interacting with a device or container. A creature would have to be right next to you to react.
- Low noise: Caused by using a Melee weapon. Creatures within the room or in hallways connected through an open door will likely react.
- Moderate noise: Caused by ballistic/energy gunfire. It is unconfirmed if different ranged weapons produce different levels of noise. Either way, at this level, you will most likely alert hallway patrols and creatues in other nearby rooms.
- Loud noise: Caused by most weapon explosions e.g. grenades and some heavy weapons. Will alert most creatures several screens away.
- Alarm*: Triggering an alarm will cause most of the floor's security bots and "patrols" to leave their assigned area, and swarm the room where it was triggered.
*Note that triggering an alarm can be fairly easy to do by interacting with a Stim Tower or Master Mind, if you are not playing the Engineer. This can be used to your advantage as it will cause Security bots to leave their "entrenched" positions in armory or repair rooms. You can then sneak around and go loot the previously-guarded room, or pick apart the security bots as they come to answer the alarm one by one.