CLICK
Does it register under conditions involving extreme duress? Yes/No/Sometimes? What are your thoughts and experiences?
*I’m practicing reloads ATM and this question occurred to me.
Are you talking about the physical “click” of a hammer dropping with no discharge, or a verbal command signalling that you are fixing something on the gun (yeah, we use it)?
I guess for the first scenario it will be situationally dependant; surrounding noise, ear pro type etc. I usually hear it, as I use Peltor Comtac’s.
Hammer fall, empty chamber/FTFi
I have not been in combat, or had to use a firearm is self defense. So the highest stress situation I’ve been in is competing at my local club, and I almost become hyper aware of what my gun is doing. It doesn’t mean I always react accordingly when a malfunction or oddity happens but it does register to me.
I can tell the difference between the hammer/striker falling and the weapon not firing and pressing the trigger and having nothing at all happen. Depending on the weapon, I can tell the difference between pressing a dead trigger and pressing a trigger with the safety engaged
During stress? Telling the difference on the square range is very easy.
I cannot say while under heavy stress. I’ve never been in combat or any kind of gunfight.
I could tell competing during Service Rifle matches. Not very stressful, but I was very focused
Never had an issue recognizing this under any training situations / scenarios including force on force. In fact I have video of myself and even students going through type 1’s and type 2’s while under high stress FoF training. Not discounting that it can happen, because I can envision situations that it may not register to the shooter, but I think for a highly trained shooter, it would take a pretty extreme situation to not recognize it. As in some type of external force not allowing the shooter to feel through their senses that the weapon did not fire.
As for real world incidents, I have only had one. It was with a bolt action rifle, so time, distance and concealment was on my side but I was extremely focused and it was a direct response and not a pre-planned initiated action. I will say that I went through my immediate action by the numbers without thinking about it.
I will edit to add that I feel that there could indeed be situations that I could find myself in where I may not recognize the situation. So I do not mean to exclude myself from that possibility.
I failed to lock a mag during an admin reload in a competition (first time shooting the gun I was using that day, and had not practiced seating a full mag on a closed slide). I noticed it immediately (M&P btw). There is a saying about the 2 loudest noises being a bang when you expect a click and a click when expecting a bang.
That said, I’ve never been in a fight for my life or a gunfight… but in competition I would expect to notice it everytime.
I would think, even under extreme stress, if you missed the click and lack of recoil, and no need to track the sight, you might notice the dead trigger afterwards. I could see it happening, but I think it would be extremely rare. I would also guess that if the shooter was extremely proficient, he would notice it more than a novice shooter. When I was really green, I did all kinds of stupid stuff - I still do, but not nearly as much. 
Appreciate the insight all. This most likely is self-inflicted case of trying to substitute cool gear and gadgets over self-proficiency.
It took a while and some getting used to, to realizing the condition when I was in training, but after so much drilling, it became automatic…“click”, check the chamber. Combat reloading was practiced over and over again during our action drills, and that certainly helped especially to minimize the chances of reaching empty at an inopportune time. Practice, practice, practice. And then practice some more.