Here is an article by Matloff that is quite similar to the original one of this thread.

http://www.nidsci.org/essaycomp/gmatloff.html

Relevant quote:

"If starships have crossed to our solar system within past eons (perhaps accelerated by solar sails and decelerated by a combination of magnetic reflection of interstellar ions and solar sails), they may have created a myriad of artificial worldlets from asteroidal and cometary material. We may live within a "Dyson Sphere" (53) of millions of space habitats, each masquerading as a small comet nucleus or asteroid.

"In the absence of directed radio transmissions from the extraterrestrials to the Earth, detection will be challenging, but not impossible. One method of detection might be a search for excess infrared emissions from asteroidal objects (5)."