I had 'nearly a fire'. It was in a Bell 47 (Sioux) at Middle Wallop during training sortie that I was informed by another airbourne Sioux pilot that there was smoke emitting from my craft. I had no instrument indications and neither i or my student could see any signs of the problem during our rapid descent to the ground. Once on the ground we made a quick inspection of the engine area, armed with the fire extinguisher and found that an oil pipe was leaking oil onto the exhaust. The engine closed down and exhaust cooled and no fire, but lots of smoke. Simple types are poorly equiped with warning systems.
The comment about water in the fire warning system of the AS355. This was a regular problem that was almost predictable and the fire warning would activate after the machine had been out in the rain for some time. Whatever I thought I always accepted that there was a fire and carried out the drills that are given in the flight manual when in flight and on the ground a quick look normally resolved the problem and a few minutes of warm engine air normally dryed out the sensor/wiring. But never had a real fire.