Use your handgun's secondary fire (on-the-left-shoulder-button) to briefly disable lights rather that shooting them. Your enemies will be less suspicious. Body are a foolproof free gift that a tricky spy is on the loose. Relocate them into spaces that guards aren't patrolling to remain concealed. The two bars up leading represent your exposure and volume. Remain in the dark, and relocate gradually, to stay clear of being found. For a simple kill, sidle up against a doorway and utilize your whistle to draw a guard over. Jump up and stab them in the trachea once they walk however.