Alright so the Gravity falls theme song has a voice at the end whispering what sounds like “I’m still here”. Played backwards, this clips sounds like it’s saying “3 Letters Back”. I’m being very short with this little explanation because virtually everyone has figured this out by now. Is this a cipher? Well, many have treated it as such, but 3 is a pretty significant number in Gravity Falls. It’s the number on Dipper’s lexicon of the creatures that inhabit Gravity Falls. It has been revealed that the three is a volume number, Gideon owning number 2. So, three letters back could potentially be referring to the first book.
Again this is just a speculation, and one that my sister discovered-not me. She was pretty pleased with herself when she came running into my room yelling “I KNOW WHAT IT MEANS!”