I have had one, with no indications except the light, caused by a lose fuel line. The vaporized fuel was sucked out the exhaust (natural ejector action routinely cools the engine compartment) and ignited by the exhaust.
They are rare, due to the way the good fittings on lines stay tight and lock easily.
In a FAR/JAR Part 29 bird (airline transport), the fire zone rules are awesome. The zone must allow a 2000 F fire for 15 minutes without compromising the safety of the aircraft, basically making the fire zone similar to the flame pit of an oil burner furnace (we use an oil burner to make the 2000 F in the tests.) That is why the zone on a 76 is all shielded with steel and Ti plates and liners, and the deck is structurally isolated from the deck below.
Part 23 birds (normal catagory) do not have this requirement, and so are much more sussceptable to fire damage.