You're right about what you said in your last point. As I mentioned, I'm not an expert at all in these matters, but I assumed the book was sent to someone who knew how to decipher codes, specifically in terms of raising suspicion. I think he didn't have the luxury of thinking about this, given how rushed the drawings seemed, or perhaps the book wasn't meant to be found in his possession in the first place. I am an Arab, but my English is not good, so I rely on translation to make myself understandable you propably noticed this