If the check results in them failing to detect you when they are spawned in they seek out the trigger point, the point where you crossed into the volume, and muddle around there. If they gain LOS to you while enroute to the trigger point then they detect you regardless.
After some quite extensive testing in A20 and earlier, this seems wrong to me.
Minor: The zeds are spawned in at some point; but it's not tied to any check in and of itself. If there's few zombies loaded in the world, they're loaded while you approach the volume. If the world is sort of full with spawns already, they're loaded in only when you (nearly) enter the volume. The latter is often the case, making it look like they're loaded exactly upon entering.
Major: The detection check when you cross the volume boundary has exactly two possible outcomes:
- the zeds continue sleeping OR
- the zeds target (Attack-Target) the PLAYER who crossed the threshold
Even if some unnamed mod kept repeating that the zeds will just walk to a spot nearby, I have Never managed to repeat that. Trust me, I've tried.
There's couple ways to get something akin that;
You succeed the volume-check, and the zeds keep sleeping. Afterwards, you wake them with a sound and then leg it.
You fail the volume-check and the zeds zoom in on you. You show yourself to the zeds (which reverts them back to normal agro mechanics) and then you leg it behind a corner / hide in the darkness at night.
I can reliably do either of the above, never have I ever managed to get volume-triggered zeds to just "walk to a spot".