After you get into the passage behind the wine bottles in the cellar --
Behind the Bottles
-- and look at all the pictures on the camera --
How do I follow up on the pictures on the camera?
-- the barrels on the right side of the cellar (next to the bottles) will suddenly have a handle you can pull on.
This passage leads to the upstairs hallway, next to Charlotte's bedroom. After you go up this way, the door apparently locks behind you, so it's only a one-way passage.
What is the point, you ask? Well, you'll use this passage in connection with the fourth weird note.
Fourth Note: "We are secrets ... "