Legitimately, part of what makes scavs so upsetting is similar to an encounter I just had.
I was completely prone on the rafter above forklits, hadn't moved in a few minutes because I was pretty sure it was a dead factory and was gonna go hoover up scraps in a little bit, just wanted to make sure everyone else was out.
Scav walks out of the area below me, I turn my head to look at him (note not my body, the head rotation mechanic) and he literally turns 180deg pointing up and looks straight at me.
I've had this sort of interaction while PMC or scav aggroed as a scav as well. Scavs will walk along looking the other way while I'm perfectly still, and suddenly turn to perfectly face me.
I get that scavs need to have some inbuilt sense of sound and checking their angles and everything – that's fine. That's great. But when there are zero audio cues and they don't naturally pan over but instead snap directly to me, it's clear this isn't what's really going on under the hood.
I'm sure we've also all experienced the scav that is perfectly still until you start aiming at them within aggro range, at which point they suddenly react to your steely gaze.
Before anyone gets upset, I want scavs to be dangerous and I realize this involves some amount of 'cheating' on the scav's part. But where it is now is really in your face about how hard they cheat.