First, NASA has confirmed that Mars does have a methane cycle. On Earth, that's connected to the existence of of quintillions (or whatever) of living organisms, from microbes to roses to whales, doing their things as they respire, pass gas (yes, that definitely matters if you have enough life forms, which we do), dump waste, and die. The Martian cycle is far less robust, only a whisper of Earth's you might say. There are non-living processes that could be involved, but the LACK of a cycle would most likely mean there is no present life analogous to Earth-type life.
Second, Curiosity, the not-so-little rover that could (we tend to think of it as desk-size, but it is in fact the size of a small car and weighs 900kg), has drilled into the surface and uncovered complex organic molecules. Again, these are not by themselves proof of life, nor are they unquestionably endemic to Mars (they could be deposit from a meteorite or comet strike), but they are what we would find if microbial Martians had been - or still were - buzzing about under the surface. The source rocks, in the Gale Crater, are believes to be about 3 billion years old.
There were preliminary discoveries on both these topics made in 2014, but this, to scientists, is much stronger evidence, derived from different locations, and basically the exobiologists are partying hard. To them, the $2.5 billion dollars spent on this mission has been justified a dozen times over.
Fingers crossed.
Below: NASA images: Curiosity: Methane cycle with possible inorganic and organic sources.
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