As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PERFORMED AN AIR TURN BACK. AT CRUISE, FL 310, ALL FAIL LIGHTS ILLUMINATED ON THE OVERHEAD CARGO DETECTION ANNUNCIATOR PANEL. NO AURAL INDICATION. SMOKE DETECTOR LOOP LIGHT ON CAPTAINS FORWARD PANEL. EMERGENCY DECLARED, REPLACED THE CARGO CONTROL SMOKE DETECTOR PANEL IAW AMM. OPERATION CHECKS OK.