This achievement is tied to chapter 6. It was right in the middle of the map, when you entered the "safe room" and there is a silver safe. So be sure to open all of those safe, and before proceed to Liumang to shipyard, don't forget to equip Ichiban's Foreman job. It'll help you out in dungeon later on.
p.s: It forced me to re-do/new games+ just because I forgot to open this creep save
