Close to the Sun

Story related, can't be missed

It's found under the trapdoor on the stage.

Chapter 6 as soon as you open the trap door on the main stage.

A fast and full guide is below:


Timestamps:

00:00 Intro
01:09 Prologue
01:45 Chapter 1: All Aboard! & The Borders of Hermes Achievements + Tube 1/8
05:40 Chapter 2: Lousy Service, Demanding Customer, Carnival! & The Fire of Prometheus Achievements
07:58 Chapter 3: Happy Birthday, Fancy Calculations, Loved Ones & The Home of Hestia Achievements + Tube 2/8
18:57 Chapter 4: Blueprint Collector & The Works of Daedalus Achievements + Tube 3/8
26:27 Chapter 5: Ada’s department memo’s & The Darkness of Hades Achievements
29:23 Chapter 6: Hidden in Plain Sight, Famous Autographs & The Drama of Dionysus + Tube 4/8
35:08 Chapter 7: Aubrey likes to Rant… & The Strife of Eris Achievements + Tube 5/8
43:23 Chapter 8: Questionable Ideas & The Flowers of Persephone Achievements + Tube 6/8
47:08 Chapter 9: Ambitious Plans & The Path of Ares Achievements + Tube 7/8
50:57 Chapter 10: Letters from the Future(Tube 8/8), Tesla’s own Notes, Faster than Electricity, The Fall of Icarus, Flawless Survivor & Keeper of Promises Achievements

Done in Chapter 6, story related and unmissable.

• Begin the chapter by going straight until you run into the Totally Not BioShock Ghosts. Keep going straight until you run into the ghosts again at the edge of the balcony
• At the balcony take a hard left and follow the wall left to a series of staircases going up. At the top of the stairs take a left and go straight
• Keep going until you reach a giant hole in the floor with an Action Icon to the left of it. Walk into the icon to cross the hole and keep going straight
• You'll run into the ghosts again, follow them to the end of the hallway where debris will block your path. Look to your right at the door labeled "P. Ivanov" and open it. Go inside and walk straight to the edge of the balcony, where you can turn left and cross the fallen girder to the next suite over
• Exit this suite via the door and take a right. You'll see the ghosts again. They'll be right outside your destination, open the door labeled "A. Archer" and go inside
• Walk to the edge of the balcony for another ghost scene. When they're done talking turn around and walk back towards the door. In the narrow hallway on your left you'll see an open book on a shelf, interact with it twice to get the Guest Elevator Keycard
• Exit the suite and go right to the elevator door. Use your new keycard to open it and go inside. Pull the lever to descend down to the theatre floor
• Go straight ahead until the hallway ends and take the door to your left. Follow this hallway until you get to the stage area. Walk to the far left side of the room where you'll see the stage and a ghost playing a piano in front of it
• On either side of the stage are stairs, take them up and walk to the backstage area. On the floor you'll see a giant trapdoor, interact with it

Achievement unlocked!