What was found
CARGO SMOKE DETECTOR LIGHT ILLUMINATED CLIMBING THROUGH 12,000 FEET. MADE AIR TURN-BACK FOR LANDING AT DAYTON. LIGHT EXTINGUISHED AT 500 FEET AGL AND REMAINED OUT. NO SMOKE NOTED. AIRCRAFT LANDED WITHOUT INCIDENT. INSPECTED MAIN CARGO COMPARTMENT FOR EVIDENCE OF FIRE WITH NONE NOTED. INSPECTED AND OPERATIONALLY TESTED SMOKE DETECTOR SYSTEM. OPS CHECKS NORMAL.