Personally the situation where I'm most likely to call someone sir is where I'm giving them a forceful verbal put down. "With due respect sir that sort of language is unacceptable here." or what have you.
I'm in the UK so also when addressing a Knight of the realm in a formal setting.
(And similarly with the leafletting, cashiering, tour-guiding and door-holding occupations I've had in the past. You want people to feel important, and you're in a subservient position, so you elevate them verbally. Basically, when you're a Baldrick.)