If you're walking down the street and a total stranger a few paces ahead of you drops something and doesn't notice, how do you get his attention?
I would say, "Excuse me, sir! You dropped this."
I'd say the same without Sir which is due to German featuring two forms of you, one formal and one personal (this also influences the verb and the first sentence).
I totally understand why it is necessary to have a term like Sir in English. All I am saying is that a term like Mister would be more appropriate because it lacks any "farmer speaks to a knight" connotations.