For example, what if brown dwarfs are in fact Dyson spheres built by a galaxy-spanning Kardashev Type II (or nascent Type III) civilization, and we're seeing their heat signatures? Yes, I can readily think of a number of things wrong with this idea, but it still merits some further consideration.
Likewise, per the various objections to METI programs, what if the putative inimical aliens showed up one day and started building a Dyson sphere around us? What the hell could we do about it?
And so on...
I'm going to finish reading Vachss now, and off to work in the morning.