Hidden Demon Chests are additional chests mostly contain medicines, magic liquids, and bullets. They are only available if you have the highest bond with Magoichi. The reason is that you need the "Green Necklace" which only available on Magoichi route.

Requirement: highest bond with Magoichi.

During the 2nd visit to Yagyu Village, you'll see Magoichi standing in front of the cave after Jubei's mother died.

1. Talk to Magoichi in front of the lake, he'll give you a Saiga Bomb.
2. Use the bomb at the cracked rock near the Lightning Dragon Shrine to your left to reveal a treasure box.
3. Open the treasure box for the Green Necklace.

Hidden Chest #1: Secret Medicine

Yagyu Village, 2nd visit, in front of the Lightning Dragon Shrine after obtaining the Green Necklace.

Hidden Chest #2: Normal Bullet x10

Onigashima 1F, inside the light wall, on the Bridge of Darkness.

Hidden Chest #3: Normal Bullet x10

Gifu Castle East Area, on the rooftop of the tower, right at the fence's entrance.

Hidden Chest #4: Perfect Medicine

Gift Castle Keep Tower, inside the green-sealed door, in front of the skeleton wall.

Hidden Chest #5: Super Magic Liquid

Gift Castle Keep Tower, from #4, it's on the way to where you got the iron horse.

Hidden Chest #6: Special Magic Liquid

Gift Castle Keep Tower, inside the room beneath the staircase where you used "Purify Charm".

Hidden Chest #7: Normal Bullet x10

Gift Castle Keep Tower, 2F, in the orange-sealed door.

Hidden Chest #8: Normal Bullet x10

Demon Cave, east path, area after where you fought the giant red demon.

Hidden Chest #9: Medicine

Gifu Castle East Area, near the mountain gate after the stairs.

Hidden Chest #10: Secret Medicine

Onigashima Passages, at the end of the hallway after the entrance door.

Hidden Chest #11: Normal Bullet x10 -

Onigashima Passages, right path, area beyond the arena.

Hidden Chest #12: Item: Medicine

Onigashima 1F, right after taking the elevator from Onigashima Passages, inside the messy room.

Hidden Chest #13: Secret Medicine

Onigashima Underground, right after defeating Gogandantess, the next area in front of the door.

Hidden Chest #14: Medicine

Airship, right in front of the exit door in the storage room.

Hidden Chest #15: Medicine

Gifu Castle Keep Tower, right after the airship crashed.