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The letter that Arya stole contains details of Robb’s movements and appears to be planning an attempt to trap him. Many have assumed that this information could only come from a spy in Robb’s camp… but not the word ‘Scou—”. That’s almost certainly indicating that scouts are the source of information regarding Robb’s movements. While it’s true enough that Tywin might well lie about his source of information in a letter, truth be told, it strains credulity that there are no outriders in the westerlands trying to keep an eye on Robb.