It's possible that Hamelin communicates with them in person, so there wouldn't be notes or anything of the sort left behind. Or maybe they're supposed to destroy the notes! I don't think it means he isn't a traitor, but it's certainly something to keep in mind.
The door wasn't even locked, was it? In retrospect, I suppose that should have been a clue that we wouldn't be likely to find anything of great importance...
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