Killing an enemy AI unit that is in the field of view of another enemy AI unit will not alert the unit if it cannot see or hear the shooter.
So in example below, (a) and (b) are two opfor units. (a) is looking to the left, (b) is looking straight at (a) from a distance of about 10 m. The lines symbolizes a wall.
If player (c) kills (a) with a silent pistol, then (b) will not be alerted.
b | +--- a c