I don’t think they can see it. They will sense you if you’re directly in front of them even if you’re not moving. I use bricks and bottles a lot to manage crowd control. You don’t have to kill anything a lot of the time but I like to try and clear an area using stealth. I love the bow in these games very satisfying for silent takedowns.
This mechanic was weird as it was nothing like the first game. On Grounded in the first game, I could almost brush up against them and sneak past, even when I was facing them. Flashlight or no flashlight. The second game, it varies which ones can see you.
What I always had to do was flash the light on them to see how they'd react. In between using the environment to hide behind just in case. Some didn't. Some you'd get a tiny response. I remember this happening in the area with the snow inside one of the buildings you had to use the rope to climb up through the roof. Hardware store? Another area that had those type of clickers was the courthouse. You'd see the change of behaviour when they could sense you and this was in the sunlight beaming through.
The only other thing I can assume is they can detect heat. Not all of them but sometimes they'd know you were in the dark or the flashlight on them. You'd see them almost ready to go psycho on that pause.
Another area I'd remember this same experience was in a very tight area with a few runners, some clickers and two big shamblers. It was dark as hell as you had to try and open a loud door. I had to take out the clickers first as they were a real threat.