Latest data and research on the Martian atmosphere has determined that the methane is produced seasonally at certain regions of Mars (and not others). Methane degradates fast in the thin Martian atmosphere and therefore its presence means that it's being replenished all the time.
There are three known processes that may naturally produce methane: two are geological and the other is life. The geological processes are vulcanism (but Mars is thought to have no more active tectonic processes) and serpentinization, a mineralization process that requires water, olivine and pyroxene.
Seasonality suggests that water temperatures and state (liquid water at whatever depths this process is taking place) is crucial to the formation of methane. The article doesn't explictly says this but it does look to me that vulcanism is not the cause in any case, also for this reason. So what we have left? A somewhat rare minerlization process that requires water... and life of some sort.
It's proverbial that "where there's water, there's life". Water is there, it's confirmed, even if it's mostly ice at surface levels, it seems. I am almost sure that there is some form of life in Mars, even if it's now restricted to some specific areas, maybe under the permafrost.