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.
Source: https://phys.org/news/2019-04-methane-mars-discovery-lot-hot.html