Blocking AI Vision
Sometimes you want to keep AI from seeing something, but don't want to block the player's view. There is a way to accomplish this.
If you want to block AI vision but not the player, make a patch and position a decal so that only the transparent part is visible (note this won't work with dirt decals or others that blend with what's behind them). Using a readable decal that has a good-sized transparent part worked for me. AI will not be able to see through this (note that this only works from one direction; decals are invisible from behind). Since decals are non-solid it won't interfere with AI or the player walking around.
I'm planning on using this to keep AI from noticing missing loot if they're coming from a particular direction. It could be used to keep AI from noticing something out a window or other uses.