This is not a metaphor. It is, in fact, rather difficult to fight invisible fires—as demonstrated by this clip from the 1981 Indianapolis 500, where fuel accidentally sprayed on a hot engine ignited an invisible, smoke-less fire on the car ... and on the racer and his crew. Everybody in this video survived, but it made me curious about what fuel they were using. Also: Why, if it's this difficult to put out invisible flames, would you use a fuel that produced invisible flames to begin with?
You might expect this was just the result of a bad decision. You'd be wrong. From 1965 until 2006, Indy cars ran on methanol, a fuel that's still used in a lot of other car racing categories, as well as in Monster Trucks. (And by the Chinese, who make it from coal.) Mostly made from the methane in natural gas, methanol was actually chosen as a racing fuel because it was safer than gasoline.
In 1964, two racers died and several others were injured when two gasoline-powered cars crashed, and the blinding smoke from the resulting fireball caused a seven-car pileup. Methanol was a solution to that problem. It burned clear, so when cars did wreck, everyone else on the track would be able to see what was going on. Bonus: You can put out a methanol fire with water alone, while spraying a petroleum fire with water will just give the petroleum something to float on, potentially spreading the fire further.
Turns out, the fire you can't see is actually safer—big picture—than the one you can. Which, I suppose, could be a metaphor for something.