Hi mandyfins, I often get the same thing when people know me and call me by my name and expect me to know them, when I have absolutely no idea who they are and if I've met them before. So like you I just smile and say hello back and hope I don't look too puzzled. If you are ever expected to know more about them, you could always say you've forgotten their name and they might prompt you, and hopefully say how they know you too.