The robot sees very low-level amounts constantly in the background, but it also has monitored a number of short-lived spikes that are 10 times higher.
Methane on the Red Planet is intriguing because here on Earth, 95% of the gas comes from microbial organisms.
Comment: This is actually an assumption, not proven fact. Oil is also assumed to be the result of the decomposition of once-living organic matter, even though it's extracted from way below the maximum theoretical extent of fossil layers.
Researchers have hung on to the hope that the molecule's signature at Mars might also indicate a life presence.
Comment: They can 'hope' all they want, but it's a false hope. See previous comment.
The Curiosity team cannot identify the source of its methane, but the leading candidate is underground stores that are periodically disturbed.
Curiosity scientist Sushil Atreya said it was possible that so-called clathrates were involved.
"These are molecular cages of water-ice in which methane gas is trapped. From time to time, these could be destabilised, perhaps by some mechanical or thermal stress, and the methane gas would be released to find its way up through cracks or fissures in the rock to enter the atmosphere," the University of Michigan professor told BBC News.
A likely source for such mechanical stress would be 'marsquakes':This, of course, still leaves open the question of how the methane (CH4) got into the clathrate stores in the first place.
Massive 'earth'-quake detected on Mars, 23 February 2012
...although thermal stress is another possibility: maybe Mars' dormant volcanoes, vents and calderas are becoming active like they are on Earth?
It could have come from Martian bugs; it could also have come from a natural process, such as serpentinisation, which sees methane produced when water interacts with certain rock types.
At the moment, it is all speculation. But at least Curiosity has now made the detection.