Individually doing atmospheric analysis for every planet in the galaxy is probably an impossible task for a civilisation confined to a single solar system. Listening for signals is something our civilisation already does. If we discover radio signals from a primitive civilisation in the next star system over there’s a non-zero chance we’d panic and try to wipe them out.
That’s the risk that dark forest theory is talking about. Maybe the threat comes from a civilisation dedicated to wiping out intelligent life that just hasn’t found you yet, maybe it just comes from your nearest neighbor. Maybe there’s no threat at all. The risk of interplanetary war is still too great to turn on a light in the forest and risk a bullet from the dark.
And while knowing this, why do we still not choose to just observe and be as quiet/ non existant as possible?
Unless they have been actively and vigorously scouting for us with FTL travel, our earliest radio transmissions, even if we assume they’re somehow still recognizable and not totally lost in the background noise of space, have only made it about 126 light years or so from earth (and honestly our very earliest ones probably wouldn’t be recognizable from very far at all, Marconi’s radio was of course pretty crude, it was our first time playing with radios, so we can probably chop a good 20+ light-years off of that easily if we’re being realistic)
Now that encompasses some 60,000 or so stars, which is a tiny speck of the observable universe, and depending on how you fill out the Drake equation that could be a whole lot of aliens out there listening, or literally no one. And only about half of them, assuming no FTL travel or communication, would have had a chance to get a response to us by now (if they even wanted to) since their response would have to travel at or below C.
If they’re in the Milky Way or nearby intergalactic space and have bothered to point instruments at us that are far beyond the capabilities we have on earth now today within the last 300,000 years, they may know that homo sapiens exist, but they’d need to be within 3000 light years to know that we entered the bronze age, and within about 200 to know that we’ve even started playing with electricity (and counting on them looking specifically at us is a real long-shot)
Parts of the Andromeda galaxy, at best, is maybe aware that Australopithecus evolved. Any further out and no one has any clue that anything really resembling humans at all is here.
Now that sort of isolation does give us a bit of security in case there is a xenocidal race that would like to wipe us out somewhere in the universe, unless we’re very unlucky we probably have a long time before we have to worry about them even knowing we’re here, and at least that long again until they can do anything about it (unless they do have FTL travel) so probably not something we actually need to be concerned about, again unless we get really unlucky the sun dying in a couple billion years is probably a more pressing concern.
I think it’s likely we’re the first and oldest advanced civilization in the universe, which means we’ll likely always have a technological advantage to the tune of 100+ thousand years head start. It’s entirely plausible that we are the future’s xenocidal species.