What was found
(CAN) A/C DEPARTED WITH NR 2 ENGINE LOOP DEFERRED AS PER MEL. AIRCRAFT TO OPERATE WITH B LOOP SELECTED. 4 HRS AFTER DEPARTURE FLT CREW RECEIVED NR 2 ENGINE FIRE WARNING (LIGHT & AUDIO), FLT CREW ACCOMPLISHED FIRE DRILL & DISCHARGED BOTH FIRE-EX BOTTLES & SHUT ENGINE DOWN IN FLT. AIRCRAFT DESCENDED TO FL 250 & PROCEEDED TO DESTINATION. FLT ENGINEER & MAINT INSP ENGINE & FOUND NO EVIDENCE OF FIRE: ENGINE GEAR BOX DETECTION LOOP REPLACED, LOOP RECTIFIED & LOOP B PUT ON DEFERRAL. WITH LOOP SELECTED OFF BY F/E PANEL SWITCH SELECTED TO LOOP B, ACTUAL OR FALSE TRIGGERING OF INDIVIDUAL LOOP SHALL CAUSE BOTH WARNING BELL & IND. LIGHT. FOLLOWING SYS IS ON MEL NO PART HAS BEEN DISCOVERED AT FAULT AS OF YET.