Methane is generated by biological activity (e.g., decomposition) and can indeed indicate biological activity. I can’t find it here, but at home (somewhere) I’ve got a paper or two from the late Thomas Gold discussing methane and other hydrocarbons on other celestial bodies.
Certainly nobody expects to find biological activity on Uranus or Neptune. (Then again, who knows?) Yet, methane presence on both of these frigid gas giants is well agreed upon.
For the record, I believe that Mars was once wet, temperate and possessing of at least, microbial life.