I think it is related to the A.I. behaviour changed in beta 83500
[83390] Fixed: Most units had maximum skill, ignoring values set in the mission (http://dev-heaven.net/issues/4483)
Played the mission with beta 83363, the alarm was easily triggered when I shoot one of the soldiers near the gas station. Since the mission is designed to sneakly infiltrate a highly garrisoned tank base, such A.I. response is more realistic to me.