Possible and probable source of Curiosity Rover's $\small\sf{CH_4}$ detection on Mars
The possible source of methane is biological, and that is what everyone is hoping for, but the more likely source is geological, produced by chemical reactions between rock and water deep underground and issuing though fissures to the surface. We are dealing with very small amounts here which are difficult to detect and measure with precision. Once out in the open, they are dispersed by breezes and decomposed by sunlight, so that could explain why they come and go. It has been suggested that methane abundance varies with the seasons, but I can't see any mechanism for that, apart from clathrates. Clathrates are a sort of ice composed of water and methane, and are abundant on Earth in the ocean and in Arctic tundra. When heated, clathrates decompose and release methane. Another suggestion is that the Curiosity rover is incontinently leaking methane wherever it goes and contaminating the samples it analyses, but I cant see any explanation for that either. I think methane has been detected on Mars, but we will have to keep an open mind on the source until more data has been gathered. My own opinion is that there has never been life on Mars, but I agree that we can't rule out the possibility that 4 billion years ago conditions were sufficiently Earth-like that Mars evolved its own primeval soup and life got started in the same way as it did on Earth, but has since been eradicated or driven underground.