How and where to find the hidden Letters
Secret Letter Locations
S:
At the very beginning:

P:
Inside this building, to the left (near ‘Another Light’ Album):

I:
Farther down the previous building, in the garden (near ‘In the Garden’ Album):

N:
In the exhibit area (on your way to ‘Afterthoughts’ Album):

M:
In this secret room (near ‘Lost for a While’ Album):

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

A:
Inside the ‘void of space’ area (near ‘Stranded’ Album):

T:
After answering the phone, look behind you (near ‘What Could Go Wrong’ Album):

The Secret
If you don’t feel like collecting the Letters and just want to see the secret:
WARNING! Jump Scare!