Ninja used to attack from their enemie's blindspots. The human eye also consists of a blind spot when the person is focused on an object or small area at a distance, their field of view reduces to an only effective 6% cone of view in front of them. The ninja of Japan used to utilize that precise time when their enemy was focused on something to move in, literally from infront of the enemy and strike, without the enemy ever seeing the ninja approach.
I'm curious of whether the developers knew of this type of blind spot in the human eyes. If they did, I wonder if they thought about adding into the game, and/or if they ever tried.
It's been said that even the Viet Cong knew this technique of using the human eye's natural blind spots to literally dissappear from an opponents field of view. Practically vanish before their very eyes.
There was an episode in the X-Files that delved into this phenomenom of exploiting the human blindspots.
Here's a few links I wish the developers would watch before they complete the game.
Ninja Sentry Removal Techniques