Thanks for trying it out !!
You're talking about the "camo index", a lower rate makes you harder to see for the guards, and it's mainly influenced by lighting and stance (standing, crouching, ramping) + movement.
I'm definitely not done with the guard IA, i need to add a lot of stuff there to make it the way i want it to be. (surprised, calling centrale, shouting to alert other guards, improving the way he looks around for the player...)