Radio signals aren't the only sign. I'd really love to see some sign of megastructure engineering, but even detecting O2 in an extraterrestrial atmosphere would be huge.
IAmBroom
Those would likely be extremely low albedo objects, so harder to detect than radio signals by many orders of magnitude.
A Dyson sphere would be virtually invisible, except for a hard to reconcile "blackbody-profile versus apparent size" ratio.