The Dyson Sphere (here: Complex and interconnected Dyson Swarm)

Dyson-style energy collectors around a distant star would absorb and re-radiate energy from the star. The wavelengths of such re-radiated energy may be atypical for the star's spectral type, due to the presence of heavy elements not naturally occurring within the star.
If the percentage of such atypical wavelengths were to be significant, an alien megastructure could be detected at interstellar distances. This could indicate the presence of what has been called a Type II Kardashev civilization.

SETI has looked for such infrared-heavy spectra from solar analogs, as has Fermilab. Fermilab discovered 17 potential "ambiguous" candidates, of which four were in 2006 called "amusing but still questionable". Later searches also resulted in several candidates, all of which remain unconfirmed.