This last note, called "Spy Note" in inventory, will come some time after you have solved two of the coffins in the crypt, and even after Harper has admitted to sending the notes. Kind of pointless, but okay.
And almost all of it is gibberish. I mean, come on -- "Something something something"? She's just not trying that hard anymore.
But there is something important here. When it says, "Up the tunnels / You must seek / The hidden ways / He does sneak," what comes to mind?
Obviously she's talking about one of the hidden passages in the basement. You could try them all --
-- but my money is on the tunnel that so far hasn't yielded anything.
After you read this note, go to the basement and go into the tunnel behind the barrels, the one that leads up to the hall in front of Charlotte's bedroom. But before you open the door --
-- look through the peephole to see someone unexpected.
After the person leaves, open the door and see what just got dropped on the floor.
It's a key, and an important one; you'll need it to solve the clocks in Charlotte's bedroom.