
I noticed in the "Summerween" episode that "What?" has been said for about 23 times.
Summerween is the 12th episode.
The numbers of "What?" in each episode could mean something.
So I guess the hidden code could something like this:
?-?-?-?-?-?-?-?-?-?-?-23-?-?-?
Also, if "What?" has never been said in an episode, it could be a "space" to the hidden code.