There is a lot to do in Deathloop, and you will often find yourself finding secrets or deadends that you cannot quite get past at the moment of discovery. Sometimes you will need to backtrack later in the game after more information has been found and more knowledge has been earned.
When you first find the safe in Colt’s apartment, you may be searching for a code to open it. In games, codes are often found written in notes or on scraps of paper. In Deathloop, you cannot find the code for that safe just yet and will need to continue on into the story.
Shortly after your first trip to Updaam where you find the safe, you will need to make your way to Fristad Rock, a cold and frozen area that houses secrets of its own. You will be there to try and find a security shack, and in there is where you will find the code.
The code in question, 4371, can be found on a sheet of paper near the desk in the security office. When you get it, Colt will remark that it was him that actually created the code to begin with, and he will be exasperated that he didn’t remember it until now. Once you have the code, you can return to Updaam, head back to Colt’s apartment, and open the safe to find out what is inside. After testing, even if you use the code before you officially get it in the game, nothing will happen.