It is only used in irony. Some cases include: 1. A person comes round every day, and you find it a little annoying. You say sarcastically: "Hey, stranger!" 2. A family member or friend hasn't been in touch for years. He suddenly calls. You say ironically: "Hey, stranger!"

It's kind of a joke we say to friends whom we haven't seen for a long time or haven't spoken to for a while.. which makes it seem like they have been "strangers" to us. But you say them to friends, because it would be weird to say them to "real" strangers. They might ignore you or think there is something wrong with you. :)

This is a very informal greeting usually used to a friend you haven't seen for a long time. It means that it's been such a long time since you last saw them, they have become almost a stranger. In most cases the greeting is made jokingly as once a person is a friend, they can never be a stranger.