Hey!

There is an extra secret that you can uncover after 100% the game.

There are Letters hidden throughout the game world. They only become visible once you've collected all the Albums and found all three parts for the Clone Boy (4 AA Batteries, Clone Boy, and Clone Boy Camera). Once you've done that, equip the Clone Boy (default key is 'E') to reveal the hidden Letters.

If you need help finding the Albums and Clone Boy parts, check out this guide:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3492145904
Please note that Albums and Clone Boy parts carry over to your next playthrough. However, the Letters do not- you must collect all the Letters within a single playthrough.

At the very beginning:

Inside this building, to the left (near 'Another Light' Album):

Farther down the previous building, in the garden (near 'In the Garden' Album):

In the exhibit area (on your way to 'Afterthoughts' Album):

In this secret room (near 'Lost for a While' Album):

Go up the front stairs of the temple, then look to the right (near 'Visions' Album):

Inside the 'void of space' area (near 'Stranded' Album):

After answering the phone, look behind you (near 'What Could Go Wrong' Album):

If you don't feel like collecting the Letters and just want to see the secret:

WARNING! Jump Scare!

If you have any questions, suggestions, criticisms, found any mistakes and so on, please let me know!

Thanks to the developers for making a good game and thank you for checking out my guide!