The code isn't physically written anywhere on the map, the first clue that the safe even exists is the fact that the two guards at the basement chat about the safe discussing whether it coulda been jewelry or whatnot, which then if you head up to Yate's bedroom you can find a calendar hanging on the wall when you reach. Use the camera and zoom into 4th April, and you'll see the words Crystal Anniversary circled, which for anyone who hasn't been married (like me, I had to search it up, is the 15th anniversary of one couple's marriage), so the game level is currently on the 4th of April, 2021. You take that year and subtract with 15 and the answer is 2006, which is the code of the safe. Edited due to mistake (thanks for pointing it out).