To get this secret weapon involves unlocking the Golden Gun Side Quest. Locate the corpse of Marietta Kyrkos in Arboretum station and pick up the TranScribe Collectible and Keycard from the body. When you listen to the TranScribe it will trigger the 'Golden Gun' side quest. Now simply follow the quest marker into the Crew Quarters. You can access this area for the first time during main mission 'Gathering Echoes'. The keycard from the corpse will let you enter a cabin with a safe. The code for the safe was obtained from listening to the TranScribe. The safe contains the Artemis Golden Pistol. Picking it up finishes the quest.