Well, sorry to switch into cynic mode here, but things like this appear whenever public interest (and therefore public funding) start to wane.
There are three possibilities:
1) There is Martian life (or possibly Terrestrial life that has multiplied from ealier probes).
2) They made a mistake about detecting the methane.
3) The methane comes from volcanism and they were wrong about Mars being volcanically dead.
Going from what we already know about Mars, 2 would be the most likely so far. Trace spectroscopy as such distances isn't very reliable. Unfortunately, Beagle-2, which had a methane detector, crashed and burned so there can't be a more reliable test.
But I'm sure now they have a good public reason to send another probe to Mars ostensibly to look for life when really its just another scientific mission boring to the layman for necessary for the advancement of science. The prospect of Martian life is the ultimate carrot for pulling along that unwilling donkey, the public, who would usually rather spend all their money on killing machines so they can wave a piece of coloured cloth around and feel proud.
