As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK FLT 100 FORM PDX: ENGINE FIRE DETECTION SYSTEM SOUNDED THROUGH HEADSET AND SPEAKER, BUT NOT ON ANNUNCIATOR PANEL. PILOT ELECTED TO RETURN TO PDX AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED FIRE LOOP AND DETECTION SYSTEM. REMOVED AND REPLACED FIRE DETECTION CONTROL BOX.