Ledger? "X has scrawled in your Ledger." Though that sounds a touch ominous. Though I like ominous.
Leaf? That could go with the scribble idea since it's a synonym for a sheet of paper. I would suggest Page, but since Clubs were called Pages, it might cause some confusion.
Desk is another possibility, with the notice being "X has left a note on your Desk." Or if you want something fancier and more directly writing-related, there's Escritoire.
There's also Mailbox and Cubby, or Cubbyhole. I think Cubby is cute. I remember in kindergarten we used to have little cubbies for all our stuff, especially secret notes we weren't supposed to pass to each other.
.
