While the lead comment of this particular thread is funny, there actually is fuel on the Moon, in the form of oxygen in rocks, and hydrogen from the Solar Wind. There is also plenty of solar energy available, for cracking oxygen loose from the rocks, and for collecting/concentrating the hydrogen. Let us imagine a Moon-girdling system of solar power collectors and hydrogen collectors, plus power transmission lines so a single large oxygen-extraction plant can operate continuously even when it is night on the Moon.
NOW imagine we aren't the only species in the Universe that does something like that. If they use Alternating Current in those power-transmission lines, then we could put radio telescopes on the Moon and detect radio waves of the AC-power frequency from distant civilizations. (Can't detect any AC from a home-world that has an ionosphere; can only detect AC from airless worlds.) If they use Superconducted Direct Current, we are out of luck with respect to detecting them. Note AC could be cheaper than SDC, but SDC will be more energy-efficient --yet stellar energy is "free" after the capital cost of infrastructure, so some alien civs might use AC and others might use SDC (and I have no idea which we might use, unless we decided to be paranoid and use SDC to be less detectable).