There is a thread on Bioware's forums that covers a lot of details about the Stealth/Detect system in NWN.
Stealth and Detect ThreadSome interesting facts:
Posted 02/23/07 18:34 (GMT) by Stanley Woo
Ok, I got a PM from fuzzy_shortyking about this, so I thought I'd scour the old BioWare programmer list for people who'd worked on NWN. After three or four attempts, I managed to snag the inimitable Brenon "Kilimanjaro" Holmes, who was able to give me the low-down on Detect Mode and all of its secret, inner workings. We tried to involve Craig "Uncanny Frankie Valli" Welburn in the conversation, but the network gods we worship were not appeased by our offering of stale Doritos and croissants. Here's the breakdown I received from Mr. Holmes, which is probably about as "official" as any info gets. He went through old NWN code and everything!
Passive (default) mode
Trap detection radius: 5ft
Trap detection rate: 6 seconds (every round)
Trap detection roll: d20
Spot/Listen roll: d10
Active (Detect) mode
Trap detection radius: 10ft
Trap detection rate: 3 seconds (twice per round)
Trap detection roll: d20
Spot/Listen roll: d20
Stealth checks
Player detects stealth: 5 times per second
Player rolls for hide/move silently/spot/listen: 6 seconds
NPC detects stealth: 4 seconds
NPC rolls for hide/move silently/spot/listen: 6 seconds
Hopefully, that will clear something up. I'm still confused. 
Posted 02/23/07 21:31 (GMT) by Brenon Holmes
Listen/Move Silently:
* Automatically cannot detect silenced creatures
* Can only detect invisible (or when you're blind) creatures within the max attack range.
* Cannot hear sanctuaried creatures
* If there is something between you and the target (including creatures) there's a modifier of +5 DC for every 40cm of thickness in outdoor areas
* In indoor areas, if the LOS is blocked and the target is within 4 tiles (dfs search) there's a modifier of +2 DC
* Area listen check modifiers
* +10 DC if you are in combat
* +5 DC if the target is standing still
* -5 DC if you are standing still
* +1 DC for every 3 meters of distance to the target
* Relative size modifiers (Tiny: +8, Small: +4, Medium: 0, Large: -4, Huge: -8)
* Favoured enemy bonuses
Spot/Hide In Shadows:
* Automatically cannot detect invisible creatures
* Automatically cannot detect anything while blinded
* Area spot check modifiers
* During Night, if you do not have a light on you (including weapon lights and darkvision) there is a +5 DC modifier
* During Night, if the target has a light on them, there is a -10 DC modifier
* For stealthed players only, there is a +5 DC if they are in the back arc
* +10 DC if you are in combat
* +5 DC if the target is standing still
* -5 DC if you are standing still
* Relative size modifiers (Tiny: +8, Small: +4, Medium: 0, Large: -4, Huge: -8)
* Favoured enemy bonuses
There might be some additional modifiers, however I can't recall any off the top of my head and without doing an in depth search through the code I probably won't find them all. Hopefully this answers any additional questions you folks had.
-420
