Something that is very clear between Thief 2 and TDM is how far you have to be away from a guard while fully lit for them to not notice you. Thief 2 is very lenient in comparison but I find the range TDM gives to guards to be borderline anal. It is something that hits me only in big city missions. The two that cause it the most for me is Penny Dreadful 3 and Crucible of Omens. Crucible more so.
So here's a compacted scenario from Crucible I will use as an example: I'm in the dark in the street and I see a well-lit door. I'm not sure whether the door is frobbable or not. I look around and decide it's safe to investigate. I find out it's a door purely taking up space and I move on.
Cool?
No. Cause as I go on and eventually make it to the Old Mill, I come across the guard which goes out on the balcony but his sword is out. I'm confused for a second but eventually I realise this guy actually saw me earlier. I reload a previous save and I watch that balcony while standing in the same lit spot and confirm my suspicions.
Now, "is that fair?" Maybe, maybe not. I could argue it wasn't fair because I couldn't hear the AI bark. I could argue it was because even at that distance, the guard can make out this hooded character slinking about. Perhaps the question needs to be asked, "Should players ever be unwarned of guards who have become alerted?"
My 2 cents is that at a certain range, a human can't tell whether you are friend or foe. How did the guard standing some 40 meters away from me, tell me apart from his mates who aren't even wearing a uniform!
I'm not 100% on what routines the AI takes, but assuming there is time allowance given for them to move to a heightened state (let’s call it time-to-acquire), then perhaps the function that gives time-to-acquire should be revised to follow a steeper curve. At 40 meters, guard takes 10 seconds, at 1000 meters, he takes a month 
