I, too, have seen some subs who seem to prefer to address everyone else as Sir or Ma'am, and I have seen contracts that instruct subs to address strangers as Sir or Ma'am. I suppose it's just to reinforce their humility, not necessarily to offer themselves to everyone they bump in to.

I know that for some people, even the very generic term "Sir" can hold a lot of meaning. But remember that not so long ago, it was standard etiquette to address strangers as Sir or Ma'am or Miss! So maybe subs are just a very polite bunch.