We've been digging around on Mars for forty years now. On Earth, where we do have life, you can't dig anywhere... from the highest mountain peaks to the deepest trenches in the sea to the desserts to Antarctica... without finding an abundance of telltale signs of life.
I would think that if Mars once did host life, that it would have traces of past life in every nook and cranny like on Earth.
The fact that there's evidence of water on Mars makes it even more likely that if there was life that we would be able to see traces of it... but so far nothing.
I'm all for continuing to look, but the more we do look and don't find anything the more it convinces me that there's nothing to find (at least on Mars).
In fact, if there is life out there in the Universe, and if life occurs out of existing materials purely at random, then life should have existed in literally billions and billions of places. Evidence of life would be hard to miss... like on Earth. Life, once started, spreads and adapts to exist everywhere... it's hard to stop and it's even harder to disguise its existence.
For now, the mysteriously persistent methane traces on Mars are seen as potentially suggestive of active life there, as is the evidence of water flows and large sedimentary rock deposits. NASA seems to think that there is microbial life on Mars, or at least evidence of it. They dare not talk too openly though because robust scientific proof requires the collection and analysis of microbial samples or fossils.