PHILADELPHIA (WPVI) -- On February 19, 2005, a young couple left a bar in South Philadelphia - and vanished.For 15 years, the family and friends of Richard Petrone and Danielle Imbo - along with investigators from the Philadelphia Police Department and the FBI - have been trying to answer one question: what happened?In this edition of 6abc True Crime, we take a look at the disappearance that has become one of the most intriguing missing persons cases in recent Philly history.Watch more episodes of 6abc True Crime HERE