She brought the book to the Secret Service with Marina's other belongings and they found the note in the book.
Not saying it was planted but there were plenty of opportunities to do so under those circumstances.
Again, why would anyone need to link Oswald to the Walker attempt AFTER he was already dead, and the investigating authorities were satisfied of his guilt? What would be the purpose of all that? The conspirators aren't satisfied with just framing him for the JFK assassination and then murdering him to keep him silent? They now have to frame him for yet another crime by fabricating a note, hiding it in a book in Paine's home, and then somehow manipulating the discovery of the note?
In doing all this, however, they fail to directly link it to the Walker attempt by making specific reference to that event in the note. The entire purpose of the fabricated note is to frame Oswald in this conspiracy fantasy - right? So why not go all out and make that explicit. "Dear Marina. Don't wait up. I'm off to shoot General Walker." LOL. Of course Oswald himself would have every reason to be coy about the contents of any such letter since it could be used later as evidence against him. So he has every reason to not be more explicit about why he had left such a note. It aligns perfectly with Oswald's purpose but not that of any fantasy conspirator.