What was found
FLT 711 FROM DEN TO TOL, AFTER TAKEOFF FROM DEN, AFT CARGO COMPARTMENT INDICATED A FIRE. EMERGENCY WAS DECLARED AIRCRAFT RETURNED TO DEN AND LANDED WITHOUT INCIDENT. ON POST FLIGHT, INSPECTED AFT CARGO COMPARTMENT FOR FIRE OR SMOKE, NO INDICATIONS WERE PRESENT. ITEM WAS PLACED ON DMI AND AIRCRAFT WAS RETURNED TO SERVICE. FURTHER TROUBLESHOOTING, RESET CB'S RAN SYSTEM TESTS OF LOWER CARGO SMOKE DETECTION SYSTEM OPS CHECKS GOOD, DMI CLEARED, PLACARDS REMOVED. AS PRECAUTIONARY REMOVED AND REPLACED CENTRAL CONTROL UNIT, OPS CHECK GOOD.