Medium has some collectibles you can dig up. One is titled Notes of a Confused Man, which refers to a secret performance called Unknown Result. It costs 40 Gamerscore, and you have to collect all 18 scores at once to unlock it. Here is their location, so you can acquire this achievement during the game.
After catching up on the grief outside Thomas’ office. Look in the garbage drawer on the right before you enter his office to find the note.
List of dwellings
Examine the broken desk in the office. Look to the right to find it on the shelf.
In the red room behind the desk. Go around the projector and its table and find a side on the floor.
Following the path of the Spirit of Grief, leads you to the locked entrance of the Living Room. Go right up the stairs and follow the pole on the left. The page lay on the floor beside her.
Good old Frank
After I got out of the locker room at the pool. Follow the narrow corridor to the wider one. Before continuing to the rubble at the end of the path, look on the right-hand page between a few leaves.
Dust and Death
After I bought a bolt cutter. Go back to the hotel lobby before opening the entrance to the lounge. Open the closed door at the back. Go to the pile of tables and chairs to get the page.
Something is broken
After the cut with Sadness on the Swing. Enter the garage at the end of the corridor and walk towards the counter at the back. Examine the file for Echo, then look for a note at the bottom of the shelf to your right.
Inside the ruins of the Red House. To the right of the mirror is a space blocked by debris. The note is on the mess.
Different types of diseases
In the basement of the Red House. Go to the front of the driveway and turn left. Follow the wall to the table and a couple of chairs at the back. There’s a note on the table.
Go back to the basement entrance. Walk past him to the left until the camera angle changes. Stand facing the wall to the left and look at the page on the shelf.
Found a way
Turn right and go to the basement. The page dead-ends on the ground.
After Moe escaped into the bunker corridor. Look at the note on the table in the room.
A relic of the past
Top of the control panel in the control room of the water pumping station.
A Fine line
In the bunker’s fuse room after we electrocuted Mau. From the fuse box, go down the stairs and turn right. The page rests on a metal shelf.
After taking the spare fuse from outside Lily’s room. Go home and grab a piece of paper from his desk.
Just me and her.
In the bunker kitchen. Look at the broken mirror, turn to the right and wait for the camera angle to change. There’s a note on the table.
Keep a distance
In Thomas’s office, while you’re looking for pieces of the mirror. Look at the shelf at the right wall after you entered.
Take a few steps in front of the board to pick up a card from the table.