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?