As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT, RECEIVED MASTER WARNING FOR AN OPEN REAR SERVICE DOOR, LT. THE PROCEDURES IN QRH WERE REFERENCED; DECLARED AN EMERGENCY AND DIVERTED TO SDF. MAINTENANCE CONFIRMED THERE WAS NO MECHANICAL DAMAGE TO THE LOCKING MECHANISM AND DEFERRED THE DOOR INDICATION.