What was found
(CAN) AFTER TAKEOFF, CREW RECEIVED A CARGO BOTTLE, LOW CAUTION MESSAGE. ACFT RETURNED TO THE FIELD. CREW REPORTS THAT THEY DID NOT DISCHARGE THE BOTTLE AND RECEIVED NO OTHER COCKPIT INDICATIONS. F/A AND PASSENGERS REPORTED THEY DID NOT SMELL SMOKE OR NOTICE ANY OTHER ABNORMALITIES IN CABIN. MAINTENANCE FOUND THE SMALL CARGO FIRE BOTTLE DISCHARGED, LARGE CARGO FIRE BOTTLE WAS STILL FULL. INSPECTED SYSTEM COMPONENTS, NO DEFECTS NOTED ASIDE FROM THE BLOWN BOTTLE. SUSPECT THAT THE SMALLER BOTTLE OVERHEATED ABOVE APU COMPARTMENT AND SELF DISCHARGED. REPLACED EMPTY BOTTLE, SQUIB ASSEMBLY.