Solution for Ninja! (Complete an Outpost without being detected)
This can be easily accomplished by loading up Sequence 3 Chapter 3 and just hit all the guards with bezerker darts from the rooftop or trees, wait til they all die, be sure the area is clear, and the keyholder is dead. Now just loot the key from his body and go open the warehouse door. The achievement unlocks as soon as you open the door and if at any point you are seen, simply restart checkpoint.
This can be done at any Outpost at anytime but I recommend this way because attempting this outside of the story you not only have to reach the door, you then have to make it outside the perimeter unseen for it to unlock. Also if you are noticed, theres no second chance and you'd potentially need to go elsewhere or wait til the warehouse is full.