Did you move at all? They hear you and on harder difficulties they hear every little thing.
Nope. I never moved but once it had direct line of sight it saw me. Even restarted some checkpoints to double check it and I could repeat the situation 15 - 20ft away. This is on Survivor for the first playthrough. I've crept up on them very slowly behind and no detection, even sat behind them 3 - 5ft but if in front, they can detect you far away facing you with direct line of sight. The spotted notification starts highlighting.
The first time I started encountering this was in the courthouse.
However, using the bloaters/shamblers as bait is quite fun luring them in situations. They're so dumb.
The area where you've to smash the sliding doors was quite intense in such a confined area before climbing the building. Threw a brick in the side window and landed as the first distraction. Got a bottle in for the lure to group them all, then lobbed in two explosives. What a mess. Then left the shambler roaming while luring it to another area to get the supplies before sneaking out over the vehicle.
In some ways the play style is reminding me of Uncharted 4, run somewhere then sneak around to let the AI settle. Watching the patterns is quite fun before and after upsetting them. Same for the seraphites/scars. Humans aint as bad with detection on survival. Except the snipers.
It is getting real intense now, descending the building last night was where I ended. The stalkers are real nasty. So unpredictable. Either rush you or they run away then rush you. They're quite wacky with the long arms slapping away, jumping gaps on the floors or sliding through crevices. I do love the cheeky way they peek around the corners in the shadows.
The worst part I'll never forget with the stalkers was in the forrest, so frightening the way the fire was as they appeared from the shadows in every direction rushing you.