|Postcard from Dyson Sphere|
Back in 1960, Freeman Dyson wrote a paper suggesting that astronomers look for artificial sources of infra-red radiation. His reasoning was simple enough - as civilisations expand technologically, their energy needs increase as well. A possible solution would be to use more of the available solar radiation by constructing collectors in space. These could, if there were enough of them, reduce the light from their star. In an extreme case, all or most of the visible light would be blocked by collectors, so the star would disappear from view.
Instead, from a distance, the heat signature of the collectors in orbit around the star would be seen instead. Hence the search for infra-red radiation.
Dyson spheres have caught the public immagination recently, but it's important to note that Dyson himself did not suggest that it had to be a single engineered structure surrounding the star. His paper suggested millions of individual and separate platforms, each intercepting a small portion of of the suns light. Collectively, they would trap most or all of it. Dyson didn't discuss the construction, just the effects, and whether or not these could be recognised remotely. In fact, a single solid structure would be impossible to build with any materials currently known to science.
The obvious implication of confirming the existance of a Dyson sphere is that it's a sign of a well established high technology space-faring civilisation. And, once you build something like this, theres no reason to restrict it's use. It's almost certain that it would develop industrial and residential areas in addition to energy production.
That's why there have been attempts to find anomalous infra red sources from the IRAS database on a number of occasions. So far unsuccessfully.