I don't usually care too much if people assign me the wrong title, but he specifically asked, then put miss instead of ms on his gadget, so I corrected him. He seemed confused and asked why not Miss? So I explained that generally people took Miss to mean unmarried, Mrs to mean married, and that Ms means it's none of your business, just like Mr doesn't announce a man's marital status. He still seemed confused and seemed to think that Miss and Ms were interchangeable. But by that point he'd finished tapping on his gadget and I was getting cold so I left it. I hope something sticks though!
To be clear, he seemed genuinely interested to get it right, I don't normally give lectures on my doorstep :)