In reply to Ridge:
Unlikely. Such a megastructure uses the solar light it catches, it doesn’t destroy it. After it’s used it it has to get rid of it; almost certainly as rejected waste heat. Ergo it must radiate the same energy that it receives from the star [*]. As the surface area of the Dyson sphere is much larger than that of that of the star, the surface will be much cooler and will largely radiate in the infra red leaving a very clear (to a telescope) signature that screams “I’m a Dyson sphere”.
[*] some small fraction of the energy may be converted to mass instead of ultimately becoming waste heat, depending on what they’re doing with it.