As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT, JUST AFTER LEVELING OFF AT FL280, THE CREW NOTED MULTIPLE CHIME AND MESSAGES FOR THE MAIN DOOR BEING OPEN. THERE WAS NO CHANGE IN PRESSURE SO IT WAS A SUSPECTED SENSOR PROBLEM SO THEY MADE AN AIR-TURN-BACK. THE MX INSPECTED THE AC AND REPLACE THE AIR STAIR DOOR PROXIMITY SENSOR HARNESS. THE AC WAS RETURNED TO SERVICE WITH NO FURTHER DEFECTS. (K)