An old trick that I first read of in a Jack Absalom book is to get a portable AM radio. Tune it off away from stations - so there is just static. Hold it near the wiring, and move the wires about.

intermittent connections etc will make sparks that the radio may pick up.

I like Ben-e-Boy's suggestion above - a bit like going back to a restore point on the computer.

I think, but am not 100% sure that there is a black plastic cylinder in the engine bay of Petrol patrols that collects fuel vapor. Mine makes the occasional noise with the engine off.