What was found
IN CRUISE FLIGHT, THE NR 2 ENGINE FIRE LIGHT FLASHED ON AND OFF CONTINUOUSLY. DESCENDED TO 300 FEET, CHECKED SHADOW FOR SMOKE, NONE SEEN, RETURNED TO BASE. ALL ENGINE INSTRUMENTS NORMAL. REMOVED NR 2 ENGINE COWL, INSPECTED LOWER DETECTOR, INSPECTED UPPER DETECTOR, FOUND LEAD LOOSE ON UPPER DETECTOR. INSTALLED NEW LOCK WASHER AND NUT. OPS CHECK OK.