There is. Melvil Dewey and John Cotton Dana created during a "meeting" with Kate Sanborn in in 1881. To insure secrecy they never told anyone else or wrote it down or even used it themselves, but legend says that there are photos in the "Extremely Salacious and Obscene" section of the Vatican Library. So yes, it exists, but nobody can show it to you.

The most commonly used secret greeting is "Want a beer or something stronger?" The secret response is "You buyin'?"

The most common secret question is "So, did they leave any money in your budget?" The secret response is "SHEEEEEEEEEE-IT! Are you kiddin'?"