Dyson Spheres

If they're old enough, they've caged their stars. We'd see flickers. We look for flickers.

Freeman Dyson's 1960 Science paper proposed searching for infrared signatures of civilizations that enclose their stars to harvest energy. Documented: the paper, and the 2015 flap over Tabby's Star (KIC 8462852), whose irregular dimming briefly made 'alien megastructure' a mainstream astronomy headline before dust models prevailed. The node marks lore's favorite scale shift: if the powerful hide structures on Earth, the truly powerful hide them around stars — and SETI's null results become evidence of better shielding.