Normal stealth checks would not be desirable in the attack volume case as the zeds are often around corners, enclosed or facing the wrong direction and the player may be making little noise which means default unskilled stealth would mostly work, yet the designers want the zeds to attack.
The one time check is to allow stealth to be useful as a skill you can put points into and allowing for variation in the way you entered the volume compared to zed locations.
The attack uses SetAttackTarget, which is a standard way of the tracking you and used in cases like when you hit them or they see you. I am reducing the trigger's call of SetAttackTarget from 60 to 20 seconds, which seems more reasonable for the attack to time out, since it refreshes anyway if they see you.