There are two main scenarios to this phenomenon. One: The first dater is only interested in the other person as a friend. Thus to them, they are only hanging out with a friend. The other 'dater' is actually interested in the other person, but knows that the person is only interested in them as a friend...at least at that moment. So while they are really on a date, it is officially called ' hanging out with a friend'.
Two: Both people are interested in each other, but don't know how the other one feels. So they go on dates, but call it just 'hanging out with a friend'.
Friend #2: John and I are going to grab some dinner and go see a movie.
Friend #1: Are you all dating now?? You know that he likes you.
Friend #2: John and I?!? Oh no! We're just friends! We're just hanging out, its completely innocent.'
Friend #1: Uh-huh. You're soo being sneak dated!