This achievement can be found in the fourth level "Italian Job"
At the start of the level walk forward and turn right into the first door you see, and then turn right again. This will take you into the main restaurant area, from here turn right and go into the kitchen. Open the second drawer to find a key with playing card symbols. Once you have this go back out into the door you just came through and walk straight into the office room.
Directly across from the door you open on the floor there is a hatch that you can now open with the key. Jump down the ladder and go to the end of the hallway. Turn left and complete the hacking minigame to open the safe & your achievement will pop.
Video guide, starts - 0:15