This is the most likely possibility. Amateur astronomers (as well as professionals) never saw an inkling of UFOs in space. So either UFOs are cloaked in space and decloaks in the atmosphere, or they come from earth.

If i push a bayesian reasoning far enough, i'll say i think weird theories involving lizard people, Atlantis or hollow earth (or illuminaties or... etc) are sightly more plausible than those involving extraterrestrial beings. Most plausible is that none of them is correct, but still.