IIRC, methane has been detected on other worlds, and would not necessarily indicate life.
http://www.astronomy.com/asy/default.aspx?c=a&id=6743
March 19, 2008
The NASA/ESA Hubble Space Telescope has made the first detection ever of an organic molecule in a planet orbiting another star. This breakthrough is an important step in eventually identifying signs of life on a planet outside our solar system.
The telltale signature of the molecule methane in the atmosphere of the Jupiter-sized extrasolar planet HD 189733b has been found with the Hubble Space Telescope. Under the right circumstances methane can play a key role in prebiotic chemistry, the chemical reactions considered necessary to form life as we know it. Although methane has been detected on most of the planets in our solar system, this is the first time any organic molecule has been detected on a world orbiting another star.