I had this on my first (and only so far) play-through of the infantry showcase mission. I flanked the two enemy watchers hiding near the rocks and stood up. They failed to see me. One was standing up, the other lying down. I fired shots (which missed) at the one standing up yet neither reacted to the sound of my gun. Only when I hit one of them did they then engage me. I started the mission and had been following my team's position and orders. Had been a bit stealthy climbing up toward the rocks (laying down as I moved closer to the waypoint), then flanked and stood right up.

Standard/default settings - haven't changed/configured anything.