3-4 sqm out of their range vision (out of the radio where they actually start chasing and attacking u) the mobs can "feel" someone so they COULD cast spells but doesn't chase u until u are near enough ( i would say 1-2 sqm out of ur 8-9 sqm of vision)
So, if no one is close enough of those parameters, they become somelike "passive mobs", a few of them slow their movements and no one of them cast attack spells at all (but probably could heal themselves or so). when using stealth rings the mob unnable to see invisibility just become "passive".
**this mechanism have another version where mobs that doesn't feel/sense no human close enough they stop moving at all, but this isn't supposed to be seen and triggers slightly different **