https://thenextweb.com/syndication/2019/04/02/methane-smelt-on-mars-but-who-dealt-it/

The discovery of life on Mars would get pretty much everyone excited. But the scientists hunting for it would probably be happy no matter what the outcome of their search – whether life turned out to extinct, dormant or extant. They’d even consider finding no evidence of life whatsoever to be an important discovery. But, as the saying goes, absence of evidence is not evidence of absence, and it will take many decades of detailed exploration of Mars to be reasonably sure that life has always been absent there.

There have been no direct observations of living organisms or fossils on Mars so far. But there are other types of evidence. One of the most often cited is the controversial detection of methane in the planet’s atmosphere, first in 2004 and then in 2014. This could have been produced by some kind of past or present microbial lifeform. However, the abundance is so low that the data remain uncertain. And in 2018, the team behind the European Space Agency’s Trace Gas Orbiter said they had failed to discover any methane.