From here


Methane was detected in the Martian atmosphere five years ago; scientists have found it is more abundant over particular parts of the planet.
It should last for only a short time in the atmosphere until it is destroyed by sunlight, and so its continued presence means it is being replenished. This suggests the methane is made by an ongoing process.
Quote Originally Posted by Michael Mumma
"The production (of methane) is likely due to only one of two possibilities. The first is geochemistry, the second is biology. That raises much interest on which one is the dominant production mechanism."
Some scientists consider it possible that microbes could have survived for aeons below the Martian permafrost layer, where water changes from ice into liquid.

and from Space

Onstott and his colleagues are building an optical device for a future rover mission that could solve the Martian methane mystery. The project is part of the Astrobiology Science and Technology Instrument Development and Mission Concept Studies (ASTID).


Methane gas is made biologically on Earth, but they're saying this doesn't rule out a geological cause on Mars, that a geological cause is just as likely.