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?
In Alastair Reynolds’ Revelation Space series, the victors of a “dawn war” far in the galaxy’s past were machines and they decided to wipe out any sentient life in the galaxy for reasons that aren’t important here, but not life in general. But by the time we came around they had degraded to the point that they weren’t doing a good job anymore and a few civilizations were just starting to slip out into space. Then they get detected and destroyed.
So the combination of wanting to destroy civilizations, but not all life and breaking down over time would allow it.
This is another example of a scary sci-fi novel needing a very specific set of circumstances to arise in order for the scary sci-fi novel’s story to work. It isn’t a plausible case to be basing any real-world decisions or science on.
It’s like trying to have a serious discussion of vigilantism and the death penalty and someone brings up Freddy Krueger as the basis for their argument.