For baryonic-matter-based technological civilizations, above a certain technology level there is precious little to no such artifact as "waste", especially waste with energetic properties of any kind. Baryonic matter is vanishingly rare in the universe compared to the size of the universe and dark matter, so I wonder if someone there is dumping radioactive isotopes whether that is another SETI plateau like we hold out conjectures for about radio signals today transitioning to low-power digital signals, and one day we'll notice the spectral output changing.