E.T.H.

Say we are being visited by UFO's, but instead of biological entities inside, there are mechanized, artificial beings. Or maybe they are just alien drones. Perhaps, the aliens themselves would be augmented versions of their biological ancestors. A race of machine-like automatrons who with advanced technology and adaptability, survive the eons, travel vast cosmological distances, all in the endless pursuit of energy consumption. Say they discover Earth, take our resources, disregard it's inhabitants, and move on to the next source of energy. They would probably be indifferent to mankind's fate. To wipe us out would be pretty easy. They might concoct a virus, or some kind of biomechanized agent to infiltrate our bodies, one by one, exterminating humanity all together. It wouldn't be at all like what Hollywood purports, what with all the war and violence. Instead, it would be a sad, ugly, anticlimactic downfall. Or what if they are just star eaters, and move from star to star, devouring entire solar systems. Given how old the universe is, there might be a great number of these mechanical aliens who out-evolved their crude biological parents.

Triangle UFO's are particularly compelling. One of the most reported kind of UFO shapes sighted. Phoenix Lights anyone?