The last on the list to look for should be Dyson Spheres and Ring worlds, any species sufficienty advance and powerful enough to produce such artifacts are unlikely tp be so inward looking.
While such things make interesting sci fi, the likelihood of any existing is ridiculously low.
Pollutants seem a better bet but the list needs serious thought, many pollutants on Earth can be produced by nature as well as man, though some volatile compounds can be produced by plants and other lifeforms so they may be of interest.
Biting the hand that feeds IT
