This is the big question posed by the French research behind these reports published in this week's Nature.
But, although the researchers gather the suspects - life, volcanism, an unknown natural process - in the drawing room like a scientific episode of Agatha Christie's Poirot, the findings get us no closer to knowing who the real culprit is.
Fred Taylor of Oxford's Department of Physics, for one, isn't clear what's new here: 'We have known for some time that the photochemical lifetime of methane on Mars is too long to be consistent with the recent measurements,' he tells us [see this 2007 Scientific American article].
'Therefore there must be another sink for methane on Mars, probably oxidising material like peroxides and perchlorates in the soil. It would actually be surprising if this were not the case, as this new paper recognises near the end.'
He goes on to point out that it's also not news that there's little chance of life near the surface of Mars, which is why all planned missions aim to dig deep into the Martian soil to look for telltale signs of biological activity.
Fred adds: 'What is interesting is that there is methane there in the first place. Whatever its source, it is probably being vented from well below the surface, and if it isn't a biological source it still must be something interesting like geothermal activity or small cometary impacts.'