I've settled down next to one of the irrigation tunnels and been using it pretty often for the past 3 days. Couple comments on this subject: 1) Lighting in the tunnel has been living own life: sometimes completely dark, sometimes bright day light, and today I found this issue combined (on the 2nd video).
2) Infected may track you from the top ground. Not just hearing your steps down there but also clearly tracking you just on top of you.
3) Irrigation tunnel materials. All 3 entrances are cleared, have rope ladder, also I've put 6 ropes and 50 short sticks on each in and out - unfortunately, it does nothing, seems unnecessary "function" at all. Check 1st video.
Reproduction steps
Go to any of the irrigation tunnels. Build rope ladder if needed. Get on the ladder and go down.
Exact build version (optional)
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Where did it happen?
Official
Is it modded?
No
Expected result
It is expected to be completely dark down inside tunnels. Infected should not track player's exact movements from the top above the tunnel.