First off, I'm not the person who actually realized this. I just saw it online, and want to get some thoughts on it.
On the cover of Mission Hurricane the lightning goes THROUGH the 39 clues logo on the front.
Historically, the only time that something punctures/messes with/changes the 39 Clues logo, it's on the last book of a series. What if- the Outcast's story finishes within Mission Hurricane? Then, the last book of all *could* be kind of like a giant epilogue, the conclusion to the entire 39 Clues series. I don't know, it's just a thought. But it would be kind of interesting. (I would probably cry through the entire book if we got a whole book of epilogue. Oh my goodness. I can't even think about it.)
Agh..Now I'm thinking about the last book. That is going to be the most emotional thing ever.... The last time that we'll ever get to see those characters. The last book to laugh at Dan and Amy's bickering, the last book of Jonah's crazy artist nature, the last book of hoping for a tiny Amian moment.
The last book.