As you progress, you can unlock talents in the Medical, Tech, Security Wings. I had One is None equipped from the Security Wing.
Go to Abilities
Go to Talents
Select a talent that relates to getting a bonus of some kind when you kill a target, such as the one I had - One is None (headshot has 50% chance of not using a bullet)
Go do what it says in the talent text, in this case, I got a headshot.