Here's the video timed to where it happened.
Now that I look at it again, I think what happened isn't that trick. I think the player just hugged the wall and it was close to the end of the light radius, so things were dimmer because of that, not any thing inherent to corners or leaning per se.
But actually that makes me suspect this may be behind the phenomenon you saw. It's very common that mappers will drag their light radii (radiuses?) out to the edge of a wall so there'd be a natural fall off there. We get kind of trained to keep the light radii as tight as possible because light radius has probably the biggest impact on performance as anything. I mean having multiple light radii overlap is a big FPS drain, and the easiest way to avoid it is have the radii hug the walls.
FYI -- since I worked on exactly this, I can let you know how the system came to be -- the reason tools, bodies, open doors, etc., don't count towards the stealth score is because of attribution. We didn't want the score counting towards other AI putting the AI on alert, so the system counts alerts when a trigger's attribution arg lists the "player" property. Of course sounds the player makes or being seen is easily attributable to the player, but seeing or hearing tools or doors or bodies will be attributed to the tools or doors or bodies, not to the last person using them; and it actually wouldn't even be easy to have the system know or remember that.
So we had a choice, either have every tool or body count against the player (even if some were clearly caused by other AI or there for plot reasons), or just count alerts carrying attribution to the player. Between those two, having it count some alerts clearly not caused by the player is just unfair on its face, so we decided to just keep it according to attribution, even if it does end up not counting tools, etc., as a rule.
So that's how it ended up that way, and I still think between those two options it's still the better option, as in not allowing clearly unfair hits. IIRC it'd be very technically hard to have tools or bodies carry a new arg like "last owned by / caused by" to wedge the player in that way, and if we can't easily do that, then there's no easy way for the game to know who used to own those tools.
But anyway, having ghosting rules beyond the stealth score, where the player can take account of such details themselves, is perfectly understandable and the way to go.