Speaking from the other side of the discussion (not related to passing): sometimes you'll just run into people who are overly critical and will notice features about you that will instantly identify you in their minds as male. and as a result they will have this ignorant and stubborn unwillingness to acknowledge your true female nature and will literally rationalize that they are forced to "sir" you because it defies their very nature.
I have cisfemale friends who have been sir'd because a guy was CONVINCED that if a girl has any sort of facial peach fuzz, SHES A MAN! Some people are just effing idiots who can't be bothered to be respectful because some arbitrary or insignificant self-morality "doesn't allow them" to acknowledge your true gender; or to put it simply, they're full of Bull-manure.
So while there are other things besides boobs that = passability, sometimes people are just inexplicably and unwavering in their ability to be dim-witted or narrow-minded. I cant for the life of me imagine how I was able to go out with smooth shaven legs, flowy hair, dressed casually female, full makeup, even with a female voice only to have the waitress INSTANTLY "sir" me without even so much as looking at my face. Its like... really?
Hell, to some people its impossible to believe that any woman could ever be taller than 5'3" so I guess a trip to Germany would cause cardiac arrest in them.
I'm sorry, im rambling now lol