These hidden codes are found in this book: Gravity Falls' Dipper's and Mabel's Guide to Mystery and Nonstop Fun! Codes are written in Bill's Cipher! Learn more about Bill's cipher at the Gravity Falls Anime group.

Spoiler Alert!

Pg. 20-21 reads; "Dipper wrote a theme song for himself and sings it in the shower."
Pg. 30-31 reads; "After nightmares Mabel meows herself back to sleep."
Pg. 94-95 reads; "Time baby is worried about Bill."

Other pages (that has Bill's cipher): 52-53, 69, 76-77, 80-81, 88-89, 92-97, 106-109, 114-119, 124-125, 132-133, 142-145, 148, and 150

That's all I'm giving the rest gets embarrassing or is just to to talk about. How funny it may seem: it seems kind of personal even for that of fictional characters. (e.g. ~ singing in the showers, meowing, hiding things, etc.) I've actually only cipher 4 things and the other two are kind of weird, gross, etc.