Methane and O2 are not in equilibrium in Earth's atmosphere and I believe both (or at least byproduct O3 & CH4) can be detected spectroscopically.

I agree completely with the rest of your post with on e caveat. Anyone who understands spectroscopic methods knows that spectroscopic measurements detect basic chemical bonds not complex molecules. That people like BD, GL, FH, & others try to claim conclusive detection of bacteria in space based on spectra is BS.

However, the combination of specific chemical bonds in complex molecules often results in a spectrum over varying wavelengths that can be interpreted somewhat like a fingerprint. The placement of the peaks and valleys as well as their relative height can suggest two unknown materials are of similar composition or likely very different. However matches are seldom considered conclusive without additional information such as the source of the material matching a typical source for the known material or additional independent analytical corroboration.

Regards,
Mark