First to observe methane in Mars atmosphere was ESA's MarsExpress in 2004: Mars Express confirms methane in the Martian atmosphere. The finding has been since confirmed by other observations, but not all experts have agreed about it.OK, rumour of a rumour here........
........but someone on the internet says that they think they've heard someone say that NASA has possibly maybe found methane.
It would be a significant finding to have a "ground truth" and isotopic composition on methane. It is possible for Curiosity to have found methane, though it's first observation was a fluke.
There are other possible sources for it, so just the mere precence of methane would not be conclusive evidence of life: http://en.wikipedia.org/wiki/Atmosphere_of_Mars#MethaneIs there any source of methane which doesn't point back to decaying organic matter?
The principal candidates for the origin of Mars methane include non-biological processes such as water-rock reactions, radiolysis of water, and pyrite formation, all of which produce H2 that could then generate methane and hydrocarbons via Fischer-Tropsch synthesis with CO and CO2.[36] It was also recently shown that methane could be produced by a process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.[37] The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface, but may exist within the crust.[38] To prove this process is occurring, serpentinite, a mineral by-product of the process would be detected. An analog on Earth suggests that low temperature production and exhalation of methane from serpentinized rocks may be possible on Mars.[39] Another possible geophysical source could be clathrate hydrates.[40]