Are you still in chapter 11? I had the same problem, but if you just carry on the game and enter the mines from main area near where the behemoth and dog are fighting, and then continue onwards and you 'll see it along the way.
A good number of the Missions between 18 and at least the late 30's are found along the path to progress the story. --- To the Sociologist, crime is deviant behavior. To the Economist, crime is rational behavior.