What was found
AFTER TAKEOFF, DOOR WARNING ILLUMINATED WITH AN ASSOCIATED, AFT CARGO DOOR UNSECURE INDICATION AT APPROXIMATELY 3000 FT. CABIN DID NOT APPEAR TO BE PRESSURIZING NORMALLY. RETURNED TO FIELD. LANDING UNEVENTFUL. ACFT TAXIIED BACK TO THE GATE WITH THE AFT BAGGAGE DOOR HANDLE UNLOCKED. CYCLED DOOR SEVERAL TIMES. NO DEFECTS NOTED. SECURED AFT BLOWOUT PANEL. PRESSURIZED AND DEPRESSURIZED ACFT IAW AMM 21-00-00-863-801/802. DOOR REMAINS SECURE. RETURNED TO SERVICE.