There are only 8 safe boxes on the level so you’ll have to locate all keys and safe boxes available. It is advised that you kill/knock out any and all enemies to help make your search easier. The keys themselves will have a bright red string that they hang from to help you see them a little better.
Key/Safe Box #1 - The first safe box is literally in the first room where you encounter your first enemy just above the table where you grab the knife. The key is just around the corner on a little table on the left side.
Key/Safe Box #2 - Just after the first safe box, you’ll encounter two guards having a conversation, one of them is behind some bars in a little room with a bright light. The second key is in this room inside a little box and the safe box is nearby sitting on a table between a different set of guards having a conversation. There is a lamp nearby and you can clearly see the yellow and black stripes on the side of the safe box.
Key/Safe Box #3 – You will eventually reach a room that’s a shooting range. Provided you still haven’t alerted anyone, the guards will be occupied with targeting dummies. You’ll know the room as it’s full of barrels tied together in sets of 4. Through the doorway straight ahead, you should see the safe box and the key is on the right side of a set of barrels on the right side of the room.
Key/Safe Box #4 – Behind Safe Box #3 is a set of stairs. Upstairs there is a path to both the left and the right. Take the left side first, and at the end of the path before it turns, on the wall is an orange box with the key inside. Further to the left from the key you’ll find a lever that opens a gated area on this level but on the opposite side behind you. Make your way there and inside the once-gated room will be the safe box.
Key/Safe Box #5 & #6 – The order in which you find these will depend on which side you search first. This is the water-filled area with the “bridge” and lots of little shacks. Search these shacks thoroughly. One of the keys will be hanging from a panhandle on a book shelf just next to a very bright light and just outside that shack (slightly to the right facing out the doorway of the shack) look for a small set of stairs leading to the level above and follow the path around. The safe box is in a little room at the end.
The other key is on the countertop inside a lighted shack near the tunnel dug into the ground that leads to/from the radioactive water. Not the shack that has a guard in it, but the one next to the guard above the tunnel watching out over the radioactive water. Move past this shack a little way and spot a metal door with a green light over it. To the right of this door is a short bridge that leads to a little shack with a guard sitting in it, relaxing. The safe box is behind him.
Key/Safe Box #7 – When you pass through the metal door with the green light over it, immediately go down the stairs to your right. On the next landing you should see an orange box on the wall, the key (among other items) is inside of it. Continue on onto the catwalk around the edge of a room filled with shallow water. Move around the catwalk counterclockwise, and take note of the open doorway on the right. Disarm the trap just inside this door and go through. To the right is a panel you can interact with that opens a cage just to its left. The safe box is inside this cage (don’t panic, the cage will eventually re-open).
Key/Safe Box #8 – As soon as you open the door just down the hall from Safe Box #7, move around to the right behind the door. Inside that room dangling from a battered bed is the final key. The safe box is on the other side of the room with soldiers patrolling catwalks above the water. It is sitting on the left side of a well-lit hallway that leads to the end of the level and is very difficult to miss.
The achievement will pop upon opening the 8th and final safe box.