the best-kept secret in the Air Force (updated)

 . . . so secret, in fact, that even members of the Air Force do not know it.  Or if they do, they do not reveal it.

I refer, of course, to AFI 1-1, paragraph 1.6.5, forms of address for all Air Force ranks, and the habit of Enlisted folk addressing one another as "sir."  It's not there;  the AFI does not permit nor require it. Yet Airmen literally of all stripes continue to do it; I've been addressed hundreds of times at this location as "sir" but thank the ghost of Hap Arnold I haven't been called out for not addressing others that way myself. 

Among the enlisted, Chiefs may be addressed in the short form as "Chief."  All other NCO ranks may be addressed in the short form as "Sergeant."  That's what I do, though usually with the Army-approved "Sar'nt."  

Yet as an E-8, I am also addressed regularly by officers as "Senior."  The AFI does not sanction that either.