I'm still working on guard senses. It's a fairly core element of the game so I'd like to get it nailed down. We're in survival mode with the baby, though (colic, nothing life-threatening) so I haven't gotten more than one or two hours done on it.
I tried plotting guard sight area:
Sight areas are only plotted for guards that you can see; otherwise you'd be able to see someone coming from around a corner. I haven't bothered to restrict the plots to the guards' forward-facing halfspaces (guards don't see behind themselves when they're moving). I also haven't adjusted the plot to show how the guards can see lit squares from farther away. Nevertheless I think I've done enough to decide not to pursue this further.
Unfortunately the fact that this is plotted only for visible guards makes the big sight areas blink in and out when a guard walks behind a pillar, for example. I think it really detracts from the mood, too. Obviously the red background is really glaring. I tried plotting enlarged ground dots instead but that was a bit too subtle.
The one thing that is similar to this which I think will work well is to have some guards carry flashlights. This would essentially be a cone-shaped, mobile light source so it would fit in well with the look of the rest.
The other thing I've been experimenting with is the use of the question mark. When guards see you I display them with an exclamation mark. I'd always imagined using the question mark to indicate a state where they were suspicious but not necessarily in full alert. The guards in Thief have a suspicious state similar to this; they will switch from their patrol mode to a slow search of the area where they saw or heard something. The idea is to soften the boundary between being unseen and seen, to give the player a chance to react. I've been trying to figure out what that state might be in my game.
In the shot above I have an experiment where if you are within a couple of squares of being spotted the guard will display a question mark. It doesn't affect his state in any other way. This sort of works but it doesn't feel all that great to me. I tried something else too but I can't remember what it was now.