Silent Assassin can be completed in a variety of ways, but the easiest and fastest way involves going directly for the target. Make sure you don't get spotted or you must restart the mission/checkpoint.
From the start go to the left side and avoid enemy sight. Work your way to the corner of the hangar nearest the plane to find a pipe to scale. At the top, shimmy on the ledge to the window and wait for the target and guard to leave the room. Once they leave, follow them and to the door and drop your gun at the entrance to the room. Now, go to the washroom and wait for them to return. The guard will notice your gun, pick it up and return it to the hangar. With only the target in the room, garrote him, drag him to the washroom and dump the body. Exit the level the same way you began to trigger end of the level and unlock this.
Completing this strategy will also unlock:
The Security Defeated achievement in HITMAN worth 20 pointsArrange matters so that a guard finds a discarded weapon and moves it into the hangar.