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?
The real question here will be if a more advanced civilization would want to make contact with humankind, or if that advanced civilization would prefer to do abductions and experiments over humans to find a way to have humankind under control.
Why would they want to make contact instead of an undercover take over the planet?
Edit: TBH I have never understood the weird concept some people have about Aliens and UFOs, I mean about thinking they are chill dudes who will come here to exchange knowledge, pay some hookers and snort cocaine with us everything shine and good, but tbh I believe they are more the kind to make experiments with us and get humankind existence gone. Supposing they have the knowledge to get here I don’t believe they would like to chill with the monkeys.