Radio emissions from an exoplanet can serve as an indicator for the presence of an advanced civilization.
Atmospherically present pollutants can be a strong indicator of industrial-like activity occurring on an exoplanet’s surface.
Gases associated with industrial activity can be produced naturally.
Natural nuclear reactors and artificial ground-based nuclear fission-related activities can affect a terrestrial planet’s atmospheric environment in analogous ways.
Anomalous, and/or excess, radiation emanating from an exoplanet’s surface can indicate the presence of an advanced civilization.
If employed, sustainable technologies such as solar panels could be detection on an exoplanet’s surface.
Assuming the sustainability solution to the Fermi Paradox holds, a civilization should become more sustainable and therefore less detectable as it becomes more advanced.