My Latin dictionary only lists ere for sir, which is how a slave would address his/her master. But how would a lower status Roman address a person of higher status - for instance, a wage-labourer to an employer, a shopkeeper to a high-status customer, any "commoner" to an aristocrat or, indeed, a slave to one who was not his/her master?
Since posting this question, I've begun reading Latin Forms of Address from Plautus to Apuleius by Eleanor Dickey OUP 2002,and the question is considerably more complex than I ever dreamt! For instance:-
The masculine domine, as far as I can tell, never occurs ... as an address from a slave to his master. ... when a title is used, it is ere/era rather than domine/domina. ... No reason for this distinction has ever been proposed, but it seems to me that slaves might have objected to a word meaning 'owner" (ie domine) because it put them in the same class as inanimate objects; perhaps they preferred erus (which is more often translatable as 'master' than 'owner') because it referred less harshly to their own status. (p 78-79)
Domine, conversely, appears to have been used between free, equal men, by children to the head of the household, and Dickey quotes Seneca - "when we run into people whose names we don't remember, we address them as 'master'" (Ep 3.1 author's translation.)
"... the contexts are polite but not subservient." observes Dickey (p 88)
Dickey even quotes such usages as domine to brothers, and as a term of affectionate respect, as above to fathers. And not just to equals/superiors.
Don't get too self-satisfied, Cinna, when I call you 'master': I often greet your slave that way too.
(Mart. 5.57* Dickey, p77 author's translation.)
On my main question - how would a slave address a superior not his master, Dickey is clear -
Slaves addressing free men who are not their masters ... use names and the same other addresses that a free man would use. Dickey p 234
Dickey makes the point throughout that convention changed over the centuries between Republic (more egalitarian?) and Empire (more deferential?), and the differences between male and female address forms - and I had no idea what a complex and fascinating subject I had embarked on!