Miller is the closest to being right, but only because he was vague, and first.
It is indeed a fuel tank breather tube. It is attached to the upper one that is on the filler neck itself. It is held in place by a metal tab under the wheel well just above the fuel filler neck. It is #16 in the diagram below.
fuel_filler_breather.png
It is positioned so that the small hole in the end on the left is facing downward. This placement keeps dirt and debris thrown up from the tires from getting in to the gas tank. It also keeps water from flowing down the breather tube in to the gas tank when fording deep water. It easily gets clogged up with debris and it is a favorite location for mud wasps to build their nests. When it gets clogged (assuming the rest of the breather system is intact) you might notice that when you are filling the fuel tank the nozzle will keep shutting off (this is for the US with the Evaporative Recovery type fuel nozzles) which can get very annoying when trying to fill a 20 gallon tank and the filler shuts off after every half gallon or so.
This is because the fuel tank can no longer 'breath'. Often the part is just plain missing, so your tank filling might be okay but you are susceptible to getting dirt in your fuel tank.
There's your mystery part lesson. Miller gets the Brownie button!
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