What was found
ON INITIAL CLIMB AFTER DEPARTING RNWY 35L FOR A FLIGHT AT APPROX 2,000 MSL, THE RED CABIN DOOR MASTER WARNING LIGHT ILLUMINATED. COPILOT WAS DIRECTED BY THE PIC & PF TO REQUEST A RETURN TO THE AIRPORT. ATC ISSUED INSTRUCTIONS FOR SEQUENCE & AN UNEVENTFUL LANDING WAS MADE ON RNWY 35L. AFTER EXITING THE RNWY, THE MS CHECKED THAT ALL LOCKED INDICATORS ON THE DOOR WERE IN FACT ENGAGED. MS OPENED & CLOSED THE DOOR & RECHECKED ALL LOCKED INDICATORS WERE PROPERLY POSITIONED BUT THE LIGHT STAYED ON. CREW ELECTED TO RETURN TO THE LINE FOR MX ACTION. ADJUSTED CABIN DOOR WARNING SWITCH.