Do you remember the Watchers on Thief - The Metal Age, those cams that were able to see you also in the darkness?
Well, it is possible to set this ability, working in the editor, on Thief - Deadly Shadows too!
Just add to your NPCS:
LightLevel -> LightLevelToVisibilityLookUp
Usually this property has nine implemented instance, that corresponds to the different prefixed light level of the game:
Their values are expressed in percentage by 1, so 1 = 100%, 0.5 = 50%, 0.15 = 15% and so on. If you set all the values in the LookupOuput parameters to 1, the NPC will be able to see you in the darkness, because LookupOuput is how many (in percentage) the NPC can see you, in differents luminosity settings.