Actually, I get what that staff member was saying. The keyword was not "AN", but "RETURN".
When you and I read it, the word "return" means going and coming back; but from the way explained and emphasised to me, the word "return" means coming back, only.
I know that we only use Oxford and Macquarie for legal purposes, but both are behind paywall, so if we look at:
-
Cambridge: "to come or go back to a previous place", so it's one way, not both ways.
-
Collins: "Your return is your arrival back at a place where you had been before." "Ryle explained the reason for his sudden return to London"
So if we were to read 'an air return ticket', that would mean a tix to come back, not going and coming back.
Does that mean we all failed English???