Scenario that I experienced.
Classic 747 crew with `real` F/E in the cruise for about 40 mins. with the APU shut down normally before take-off.
APU fire warning occurs. F/E tests both fire loops which are OK so order given to fire the APU bottle.
Fire light goes out ................. for about 2 minutes then comes on and stays on!
En-route diversion carried out with fire warning still on until short finals.
Maintenance find that the two fire wire loops which should have been run parallel to each other in the bay have been crossed over! Thus the vibration in flight rubbed the insulation off and gave BOTH a fire warning AND a valid wire continuity check! Sometimes you just can`t outguess bad servicing.