Scientists consider the discovery of methane in the Martian atmosphere to be one of the strongest indicators of habitable conditions for life. Methane can be produced through both biological and non-biological processes, but it degrades very quickly - it should disappear within 300 years in the Martian atmosphere. So what's producing it? The last period of volcanism seems to be millions of years ago (Olympus Mons was active 100 million years ago). That's doesn't necessarily mean life, though. There could still be volcanic vents gassing methane which haven't been discovered.