I had this whole post written up and then realized I could sum it up more concisely. You're welcome!
I use the honorific 'Sir' in my relationship, as 'Master' has always felt unwieldy/unnatural for him. We're not involved in the kink community offline, and he's not involved in it online, so it's not really something we might encounter. I think if we were to be put in that situation, he'd chuckle and pull up his "Aw, shucks" southern (US) accent and say, "Just call me [his first name]," to put the submissive at ease. I'm very secure in my relationship, so if it turned out that the familiarity was intentional (Maybe flirting, for example), I'd let her knock herself out trying. Honestly, Sir would have it handled before I needed to speak up.