Advanced alien civilizations could be out there, but currently we are searching for the wrong signs of life. Richard Carrigan of Fermilab contends that we should try looking for the telltale signs of massive star engines.
In his recent paper "Starry Messages: Searching for Signatures of Interstellar Archaeology," Carrigan - who is a physicist at the Fermi National Accelerator Laboratory outside of Chicago - suggests that listening for radio signals ala SETI may not yield the close encounters we're looking for. He proposes that skywatchers search for possible industrial and refuse products from spacefaring civilizations. This means inspecting the night sky for light and nuclear pollutants as well as massive solar engines known as Dyson spheres:
The Dyson sphere conjecture speculates that a planet could be purposely broken up to form a heat absorbing shield around a star to provide more useful energy. One fanciful model of a Dyson sphere would be a star enclosed in a shroud of solar-cell calculator chips.
How would we notice a Dyson sphere? Well, for starters, they're pretty damn big.
The mass of a Jupiter-scale Dyson sphere would be 2*10^27 kg. Earth's biomass is in the 10^15 kg range. The largest ocean liner is 1.5*10^8 kg while the immobile Great Wall of China is on the order of 10^12 kg and took 2500 years to build. The International Space Station weighs 2*10^5 kg so that it is a factor of ten billion trillion smaller than a Jupiter-class Dyson sphere. In short, a Dyson sphere is a large object!
Even though the sphere would collect a star's light, there's a chance we could still gauge the infrared light. Here is Carrigan's original paper. New Scientist has a nifty breakdown of Carrigan's ET-finding methods.