YOU MUST COMPLETE THE MAIN MISSION - THE RESCUE before you can open the door. I've searched a lot for this code but in the end I've find it in some note. The game makes it realy hard to find the code. You have to search a bit. Go down the stairs on the north side of the market than go to a dead end. It’ll be a closed garage door with a hidden keypad behind a sign. Code will be 605185 .