It's too late to edit my post, but the point of the CO2/methane thing is that if we found a planet that had lots of both CO2 and methane in its atmosphere, that

It's too late to edit my post, but the point of the CO2/methane thing is that if we found a planet that had lots of both CO2 and methane in its atmosphere, that is an atmosphere that exists in a non-stable equilibrium, which implies life.