The foo fighters of WWII - now THAT, scientists could try harder to explain. :-) Allied aircraft pilots in World War II described "various UFOs or mysterious aerial phenomena seen in the skies over both the European and Pacific theaters of operations ... Formally reported from November 1944 onwards, witnesses often assumed that the foo fighters were secret weapons employed by the enemy." So, "mad German scientist" would be my first guess. ETs would be my last guess.

The Robertson Panel explored possible explanations, for instance that they were electrostatic phenomena similar to St. Elmo's fire, electromagnetic phenomena, or simply reflections of light from ice crystals.