What was found
APPROXIMATELY 20 MINUTES AFTER DEPARTURE, AT CRUISE ALTITUDE OF 7500', PILOT LOOKED AT ANNUNCIATOR PANEL AND NOTICED THE "DOOR UNLOCKED" LIGHT. PILOT NOTICED THAT THE AFT DOOR MAIN LATCH WAS NOT SECURE, THE SECONDARY LATCH WAS HOLDING THE DOOR, AND THE DOOR WAS SLIGHTLY OPEN. PILOT ATTEMPTED TO PULL THE DOOR CLOSED WITH THE SECONDARY LATCH, CAUSING THE SECONDARY LATCH TO BECOME UNLATCHED, RESULTING IN THE DOOR DEPARTING THE AIRFRAME. AIRCRAFT WAS SECURED AND AN EMERGENCY LANDING WAS CONDUCTED WITHOUT INCIDENT.