Primarily as an exercise, I thought about that question for a while, and here is part of my answer in a Bloomberg column:

If you know you are being watched, what exactly do you wish to buy more of? I would bet on defense stocks to rise, whether or not there is much we can do to defend ourselves against this alien presence.

Of course investors could not be sure that these alien drone probes will merely observe us forever. They might be observing with the purpose of rendering judgment. If they are offended by our militaristic tendencies, the quality of our TV shows and our inability to adopt the cosmopolitan values of "Star Trek" over the next 30 years, maybe they will zap us into oblivion. But that kind of systematic risk is hard to insure against. After such an act of obliteration, neither gold nor Bitcoin will do you any good.

My main prediction is that alien UFOs will be bullish for the dollar. The U.S. government seems most closely connected to the UFO phenomenon, for whatever reason. (Maybe its pilots fly more sallies and record better data?) In any case, if alien UFOs become more likely, an informational advantage would accrue to the federal government. And the dollar already has a tradition as a safe haven currency...

Most of us would get used to the idea of alien presence without quite believing in it. As The New Yorker makes clear, many Americans believed in alien-origin UFOs after World War II, as did many American policymakers. It might have spurred greater interest in the space program and science fiction, but it didn't affect most aspects of American life, nor did it seem to drive markets.

Never underestimate the capacity of markets, like humans, to adapt. Just as many of the strangest parts of our lives can come to seem normal, so Wall Street can find a way to do business with just about anybody — aliens included.

I do full, literally mean everything stated in the column.  But the piece also has (at least) two esoteric meanings -- can you guess what they are?