First, find the film roll, which can be found on a table in the Workroom in the sewers. Use the T-Bar Handle to open the Valve in the Lower Waterway (Lower Sewers), which leads to the Workroom Lift. Take the lift up to find the film roll on a table. Proceed through the Worker's Break Room, and you will be able to backtrack all the way to the Police Station. Go to the Dark Room to develop the film, and you will get two pictures of where to find hidden items. Those places are the Press Room (Police Station 1F) and S.T.A.R.S Office (Police Station 2F). Find the two hidden items to get the "Treasure Hunter" achievement.