They refer to the AI as "analog AI". They'll have a number of characteristics that are modified on the fly instead of having a simple alert or neutral as you find in most games. One example is that two guards talking to each other will have narrower "vision cones" and their hearing will be duller in comparison to a guard patrolling on his own. Light, mental state, ambient noise ect. will all impact how the AI reacts.
They're trying to avoid having the player feel like the AI is cheating. One way they're doing that is by rarely spawning new enemies, and when they do it'll only be because an alarm went off. When they do spawn they'll try to make it realistic such as having reinforments come through the backdoor of a mansion instead of just magically having them pop up near the player.