What was found
UPON TAKEOFF, IMMEDIATELY AFTER THE GEAR WAS RETRACTED, AN EICAS MSG CAUTION LIGHT FOR, MAIN CABIN DOOR, CAME ON. PILOT COMMUNICATED TO TOWER TO RETURN WITHOUT DECLARING EMER. WHEN TRANSFERRED TO APPROACH, PILOT DECLARED EMER & WAS GIVEN PRIORITY TO 25R & LANDED WITHOUT INCIDENT. ON GND, THE DOOR WAS OPENED & CLOSED SEVERAL TIMES. EACH TIME, THE DOOR IS FOUND CLOSED OR LOCKED & LIGHT EXTINGUISHED. THIS MATTER WAS RELAYED BY THE FLIGHT CREW AS THE CHECKLIST FOR THIS ABNORMALITY IS INSUFFICIENT & AMBIGUOUS, POOR DOOR WARNING CHECKLIST. REPLIED THAT THEY ARE GOING TO SUBMIT A CHANGE TO THE CHECKLIST.