A few thoughts

[Destined]

This problem may also be due to the system with which the alert state is raised:

 

When this counter exceeds a certain amount (alert_thresh#), the AI will reach the next alert stage. In order to prevent the alert level to ramp up too quickly there is a grace period (alert_gracetime#) during which a new suspicious stim will not further alert the AI. This period can be termintaed if the new stim during the grace period is too strong (exceeds the former addition to the alert level multiplied with the "alert_gracefrac#" value) or if there are too many stims in quick succession (exceeding the alert_gracecount#).

I think this system (that I personally find quite elegant) prevents you from sprinting to a guard on loud floor as each step you make that is closer to the guard is percieved louder as before and, consequently, wil quckly exceed the alert_gracefrac value and/or the alert_gracecount. Thus, the alert will jump up pretty quick. One thing that just occured to me: some people might not consider that the high alert state is reached first and then the animation for drawing the weapon plays. Maybe even another animation has to be finished first. As a consequence the AI is already immune (due to a high alert state) before the player can see that the AI is in this state.