Secrets for Sekiro: Shadows Die Twice covers aspects of the game such as mechanics revolving around hidden and hard to find locations and items.
There is a secret door at senpou temple, not far from where you find the white pinwheel. Same building as one of the smaller centipede giraffe enemies.
dispell the demon at the hag where she says make monkey dance to receive different items
There's one in Ashina castle, right where you see two samurai presumably discussing war tactics (the area where you can hook up through the broken ceiling)
There is one in Hirata Domain near the old woman (just before the Lady) on the left behind a panel with paint on it
One secret, the game is hella hard!