1. Open the ArmA 3 Editor
2. Create a NATO unit "Man (Recon) > Scout" - Player
3. Create a CSAT unit "Man > Soldier" - Non-playable; create him about 50 meters away and make him face away from the player unit.
4. Preview mission
5. Switch to your silenced pistol and then lower your weapon
6. Go to walk speed and crouch stance, and begin approaching the CSAT soldier from behind (your footsteps are silent)
7. Once you've come to about 10-15 meters the soldier will somehow be aware of your presence. This also occurs at night, even if the soldier does not have NVGoggles or acc_flashlight attachment.