i can't think of any case when this would be used other than when addressing someone (so this should be a person/persons, it even sounds awkward when used with a dog/cat, because this immediately establishes a closer contact, in most cases this shortened form is used to quickly address someone, when attention must be drawn immediately, hence this short form).... There might be some similarity here to when you say Bob! instead of Bobby! instead of Robert... but this is not the exact similarity... I think functionally this is used to address quicker (we always tend to save milliseconds in real communication) and to immediately establish contact.