I'm referring to the mechanic where the enemy gets suspicious and turns towards your stealthed character but doesn't attack (unless facing you gives him enough of a detection boost that he sees through your stealth). If he doesn't end up fully detecting you, he turns back to his previous facing after some number of seconds (see video, below).
Video of it happening at 6:24 (in Classic, not vanilla, but I don't think that matters):
https://youtu.be/1GrSZdrrZ74?si=guQ_11Z99TnEYyUF&t=384
I'm also including a screencap of weird, janky behavior that happens when you approach at an angle where the enemy should be able to see you just out of the corner of his eye. Is this a buggy attempt at implementing partial detection, or is it unrelated?
(the enemy would move towards me for half a second and then reset, which is why I'm behind him in the pic)