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?
Either they think they are the baddest guy around, which will eventually be wrong, or they worry about what others will do. Destroying a biosphere is not a quiet event. Anyone with the ability to do so also has the ability to monitor for that. So if you take out someone else you make yourself a target.
Not if you redirect a few comets. Depending on what their travel to and from looks like, we might not even notice an alien ship setting our destruction into motion.