I recommend that you start by exploring the mill's vicinities thoroughly, by jumping down and climbing the ladders, wherever possible. Taking these steps will allow you to acquire additional items, like the Bracelet - 20G, a Flask - 4G, a Syringe - 6G, an Ashtray - 3G and a Coin Purse - 8G. After you have given this area sufficient exploration, return to the main level and go for the mill. Walk over the crates and reach the locked opening in the mill's wall, at which there is a button. Press the button and enter.
Climb to the ledge above and take a turn to the right. You need to be very careful here, because there are traps around and they are activated by means of pressure plates. It is worth noting here that you can locate the traps in the Focus mode and that they are marked in red. If you cannot use focus, simply take a careful look around and take your time while doing that. Walk to the wall on the left and find the interactive switch that deactivates the first set of traps.
You can proceed now, reach the ladder. Continue the linear walk and watch out after you reach the stairs. Also here, you need to locate the interactive switch on the wall on the left, which deactivates the second (and the last one) set of traps. Climb to the very top, examine one of the supports on the left and press the last interactive switch. This will result in opening the nearby manhole. Approach the hole win the floor and jump in.
You are going to experience now some sort of a vision, where you will have to follow Erin and listen to the conversations with her. The entire walk is very linear and also, you will receive a notification, if you walk off the pre-determined path. After the screen flashes again, resume your exploration of the surroundings, by crouching whenever necessary. Your task is to locate three posters and rip them off the walls.
After you return to Erin's hideout, focus on finding a poster that is identical with the one that you have examined during your visions. Pressing the button will open access to the hidden stash, where you find a Medallion. You can leave the mill now, and the easiest way to do that is by forcing one of the nearby windows open. Return to the South Quarter [South Quarter / Riverside - Eel's End] and wait for further objectives.