The Dark Pictures Anthology: House of Ashes

There are 50 secrets found throughout the game, some of which requiring a specific chain of events to access them. This is the full list in the order in which they appear in the game, broken up by the game's chapters:

The Briefing
Secret 1: Bandit Briefing - After the mission briefing, you'll have five minutes to explore before a cutscene begins. Behind Merwin, you'll find a coffee cup you can interact with and move to find the secret.

The Raid
Secret 2: Mythology Book - When you first play as Salim, head into Zain's bedroom and the book will be on his desk. Make sure to grab this before interacting with the bed.
Secret 17: Pagan Effigy - After securing the farm house, Rachel will begin interrogating one of the men (presuming you left them alive). Choose the Head option as Jason, and he'll inspect one of the effigies on the wall, revealing the secret.
Secret 7: Gas Mask - After the first interrogation at the farm house, Rachel will have Jason escort her to another building. Head inside after speaking to Nick, and in the first room on your left, you'll find a basket filled with gas masks.
Secret 47: Punctured Skull - When you first fall underground, you'll begin playing as Nick. Make your way through the tunnels until you find Jason. When you regain control, go around the right side of the giant hand statue, where you'll find the secret.
Secret 18: Pazuzu - When Eric and Rachel first meet up underground, you'll climb around a door and traverse a narrow path. On the other side, this statue will be on the ground nearby.

The Temple
Secret 32: Curse of Akkad - When Rachel and Eric split up to look for fuel for the generator, head to the pile of rubble with the flare on top of it, and then head in the direction away from the giant set of stairs. You'll find a desk with a letter, make sure to flip the letter over and inspect the back.
Secret 12: Cuneiform Tablet - When you're exploring the temple, either before or after refueling the generator, locate the big tent where the fuel is located. Just behind the tent, you'll find this secret on a crate.
Secret 34: Expedition Members - When you're exploring the temple, either before or after refueling the generator, find the big tent near the center of the room. Inside you'll find a desk on your left, with the secret on top.
Secret 35: Aline's Letter - When you're exploring the temple, either before or after refueling the generator, head towards the giant set of stairs. Just to their left, you'll find a desk, interact with it and move the coffee cup to find the secret.
Secret 24: Randolph's Journal 1 - When you're exploring the temple, either before or after refueling the generator, find the big tent near the center of the room. Inside you'll find a desk on your right, with the secret on top.
Secret 44: Dynamite - When you're exploring the temple, either before or after refueling the generator, head towards the flare in the middle of the room. Right next to it is a big sand pit, with an interaction point on the far side from where you're standing. Interact with that point, and make sure to use the right analog stick to look to the left of the detonator for the secret.
Secret 20: Creature Features - When you're exploring the temple, either before or after refueling the generator, head up the giant stair set at the back of the room, and then enter the room to your right. Follow the corridor to find a room with the secret on a desk.
Secret 42: Reference Book - After reparing the generator with Jason and Nick, you'll investigate a new corridor. Immediately after regaining control, and at the end of a corridor you'll find a book. After initially examining it, you'll open the book, and you'll need to flip the page for the secret.
Secret 21: Star Map - After the QTE with the tripwire, Jason will radio Eric. Once you regain control, this secret will be just up ahead, to the right of a crate you move to exit the room.
Secret 13: Randolph's Journal 2 - After the QTE with the tripwire, the secret will be on the ground more or less in front of you. Make sure to get this before interacting with the crates to leave the room.
Secret 29: Holed Stone - After defending yourself as Salim, the next time you play as him, you'll find yourself in a dark corridor. Walk forward until you see the secret on the ground in front of you.
Secret 14: Randolph's Journal 3 - While playing as Nick and searching for Joey, you'll come across a puddle of blood. Once you regain control of Nick, head to the right, and you'll find this secret on the ground. Make sure to flip the page over.
Secret 5: Staked Woman - After you return to the temple and are tasked with fortifying it as Jason, you'll head through a door that's being guarded by the machine gun. Head through this corridor and you'll arrive at a room with a hole in the center. Head to the left, and you'll find a corpse by the left wall. Interact with it and move the right analog stick to find the stake.
Secret 6: Bloodless Corpse - In the same room as Secret 5, on the opposite wall. When you enter the room, head to the right until you find the corpse by the right wall. Make sure to use the right analog stick after interacting with the corpse to inspect its face.
Secret 23: Randolph's Journal 4 - In the same room as Secret 5, continue past the corpse on the left to find this secret on the ground.
Secret 31: Cylinder Seal - Head through the room containing Secret 5, and in the next tunnel, you'll find this secret on a crate to your left.
Secret 48: Ancient Helmet - After regaining control of Rachel at the bottom of the pit, continue forward, hugging the right wall until you find the secret on your right.
Secret 11: Plastic ID Card - After regaining control of Rachel at the bottom of the pit, head forward while sticking to the left, and the secret should be right in front of you.
Secret 33: Randolph's Journal 5 - After regaining control of Rachel at the bottom of the pit, head forward, staying in the center of the chamber. You should find this secret in front of the second pillar you come across.
Secret 8: Science Notes - After making a torch as Rachel, head forward until you see the secret right in front of you.

The Signal
Secret 22: Finds Catalog - When you regain control of Nick after the climbing section with Rachel, you can find this secret on a crate near where Nick starts.
Secret 49: Battle Tablet - When you regain control of Nick after the climbing section with Rachel, head towards the stairs in the background, and then take a hard left into an open area. The first thing you can interact with in this area is the secret.
Secret 9: Vampire Speculations - In the same open area as Secret 49.
Secret 39: Infection Symptoms - In the same open area as Secret 49.
Secret 25: Randolph's Journal 6 - When you regain control of Nick after the climbing section with Rachel, walk towards the stairs in the background, and at the foot of the stairs, take a hard right. Follow this path to the secret.
Secret 3: Expedition Map - When you regain control of Nick after the climbing section with Rachel, you'll see a tent nearby with a light on it. Head over there and inspect the map on the far table.
Secret 50: Ritual Tablet - Once Salim and Jason reunite after holding open a door, you'll enter a large room. Head to the right, and this secret will be sitting on top of something.
Secret 16: Randolph's Journal 7 - In the same area as Secret 50, towards the back wall in the dark.
Secret 28: Parasite Notes - In the sickbay that Jason and Salim find, in the center of the room on a desk.
Secret 46: Autopsy Notes - In the sickbay that Jason and Salim find, this secret is on a desk in the corner of the room.
Secret 30: Black Onyx - In the sickbay that Jason and Salim find, find the door on the left wall that exits the room, but don't interact with it. Just to the right, you'll find an alcove containing the secret.
Secret 26: Randolph's Journal 8 - In the sickbay that Jason and Salim find, you'll find an open cell on one wall (not the room exit on the left). Head inside the cell and you'll find the secret.

The Ancient One
Secret 40: Finds Ledger - After exiting the elevator with Jason and Salim, the secret will be nearby on a desk.
Secret 10: Cocoon - After exiting the elevator with Jason and Salim, walk forwards until you see a glowing column on your right. Interact with it, and then use the right analog stick to inspect the top of it for the secret.
Secret 4: Pocket Watch - After exiting the elevator with Jason and Salim, head forward, hugging the left wall. You'll eventually come across a corpse sitting on the ground, and just past it, this secret.
Secret 15: Randolph's Journal 9 - Just after Secret 4, near the left wall.
Secret 41: Mysterious Artifact - When Jason and Salim start exploring the city, eventually you'll reach the end of a path, and you'll have the option of dropping down or walking out onto a kind of runway. Head to the runway, and a cutscene will play. Once it ends, interact with the thing at your feet.
Secret 37: Ruptured Skull - When Jason and Salim are exploring the city, eventually you'll climb onto a round platform with a hole in the center. Head around the right side, and you'll find this secret.
Secret 27: Fossilized Lifeforms - When Jason and Salim are exploring the city, you'll reach a point where you have to navigate a corridor without stepping on anything (you'll know you're at the right part when you see a cutscene of Jason carefully stepping over something). Make it to the end without waking the monsters and you'll drop down into a new area. From there, head right and interact with the skeletons on the ground, using the right stick to look at the skeleton on the right.

Strange Aeons
Secret 43: Mary's Notes - Once you regain control of Rachel in the star chamber, this secret will be on a nearby stack of crates.
Secret 45: Van Huyten's Confession - Once you regain control of Rachel in the star chamber, this will be by the glowing wall in the back.
Secret 19: Creature Life Cycle - Once you regain control of Rachel in the star chamber, you'll find this secret on a bulletin board near the water.
Secret 36: Randolph's Journal 10 - Once you regain control of Rachel in the star chamber, this will be on the backside of the thing Salim is standing by in the middle of the room.
Secret 38: Alien Parasite - To get this, you'll need to follow the same steps required to earn The Thing . Once Eric uses the UV wand to save Rachel, search the holes in this room until you find the parasite. Complete the QTE, and then inspect the parasite on the ground for the secret.