We’re just barely getting to the point where we could detect Earthlike oxygen levels on another planet (I’m not sure if we’ve succeeded yet, but that’s the goal). Trace substances like CFCs would be far, far more difficult.
Would an abundance of free O[sup]2[/sup] be indicative of life of some kind (not necessarily industrial or even intelligent)? Is it only possible due to some ongoing process like photosynthesis?
There is one situation where there could be free oxygen and no life. A planet covered with ice, water or some combination of them and a star that produces lots of UV light. The free oxy would come from photodisociation of water molecules where the hydrogen escapes the atmosphere. It has to be pretty much all covered with something that doesn’t react with O[sub]2[/sub] i.e. water and/or ice. Extensive amounts of rocks would be a oxygen sink, so lots of land is out.
I would expect such a situation to be largely self-limiting, since the free oxygen will form an ozone layer that will block most of the UV. I don’t know what oxygen level it would more or less stabilize at, but I suspect it’s below the 20% we have here on Earth.
But yes, it being a strong (though not perfect) indicator of life is why we’re trying to be able to detect free oxygen. Though of course, one could also have life without oxygen, and we have no idea how common that might be.
Using our current equipment, and assuming we are pointing it in the exact right direction…
We would maybe detect TV broadcast carrier wave at 4 ly… definitely not enough signal to actually receive the transmission.
We could detect military radar stations at almost 30ly, in part because they are strong but mainly because they are quite directionally focused.
And the pulse from the Tsar Bomba would be detectable over 500ly away. But would look like a natural event or a sensor glitch.
Unfortunately, space is huge, and the era of energetic broadcasts has passed already.