This Achievement requires you to unlock 15 safe boxes throughout Metro 2033 Redux.
The 15 safe boxes
DO NOT have to be 15 different safe boxes, I found one safe box and kept opening it by loading the same checkpoint 15 times.
The easiest place that I found to farm this is in the chapter "Lost Tunnel".
In order to open safe boxes you require a key. A key can be found on a dead body around where your partner warns you about the tin can alarm systems.
Thanks to SDREW44 for the info about the key!
When you encounter your first group of human enemies, there is a safe box in the back next to a sitting guard and a glowing mushroom. Simply sneak past the guards (or kill them, your choice) and open the safe box. Pause, hit Load Last Checkpoint, rinse and repeat.
Below is a video if anyone needs a visual reference of the location and the process of farming the achievement.
Good Luck