They did say that doors/walls dampen sounds, that could have something to do with it.
The example I used involves said unaware guard just standing there after the player shot another guard and stabbed him. The unaware guard was like 5 feet away and he wasn't behind a door either. There is no door to balcony its just a wide open walkway.
Another example would be after killing a target two guards rush into the room shouting and
one of them fires their gun into the ceiling...while in a open hallway or lobby or whatever. The player stops time and proceeds to blow them both up with
a landmine. Then the player goes upstairs and what are the two guards doing up there? Having a conversation like everything is normal. You'd think they come downstairs and investigate all the noise especially after the explosion of the landmine.