grounded27: I have an interesting tale to tell....
situ: 747 Classic in cruise about one hour into the flight when we get a fire warning on the shutdown APU. F/E does a fire test on both loops and confirms that the warning is genuine. Fire bottle discharged and warning ceases ........... for about two minutes and then comes on again continuously! Diversion initiated to nearest airport. Warning was on all the time until about 500ft. on finals when it went out. Landed with fire services in attendance who then confirmed that no fire appeared obvious.
Engineering subsequently discovered that the twin fire loops that should have run in parallel about the APU bay had been fitted so that at one point they crossed over. This led to the insulation being abbraded giving a valid fire warning + a valid continuity check!
Moral to the story? You cant predict everything! There is generally one course of action that is the safest you just have to realise what that is!