[CamperBob2]

Yes, really true. Look at how hard it was to establish the presence of the PH3 absorption line on the planet right next door. We are not able to make that kind of observation on extrasolar planets yet, at least not beyond very rudimentary levels. Hopefully someday, but certainly not now.

Admittedly Earth's oxygen signature would be a lot easier to detect than a minute quantity of PH3 on Venus. But we wouldn't have been able to look for that either until just a few years ago. Even to a seasoned RF engineer, ALMA and JCMT are indistinguishable from magic.

[TeMPOraL]

Fair enough. What I'm saying is that detecting anomalous chemical composition of exoplanet atmospheres seems almost reachable with our current capabilities, and - unlike radio signals - is an indicator we can't really hide. So aliens slightly more advanced than us could be looking at that.