No, they don't find that evidence... but they muck up the search for it pretty early.
One of the earliest terraforming ideas and attempts in Red Mars involves dropping wind turbines on a bunch of spots that will generate heat, and provide an (extremely slow) warming affect on the atmosphere to contribute to the general altering of the environment. It's not a very effective idea, but they assume that every bit will help over a long period of time.
Well, taking that one step further, one of the scientists loaded all of the turbines with bacteria samples that would replicate in Mars environment and do more of the same. This would inadvertently destroy most possible chances for discovering independent life on Mars, because now the environment is tainted and they would have to sort out Earth bacteria from Mars bacteria. He rightly gets chewed out on it when he's found out... but it fails to matter very much in the long run, as humans start coming to colonize in large amount long before Mars is terraformed anyway.
As Mars is colonized there is a determined faction that wishes to keep the original environment as pristine as possible, and in fact ends up wielding considerable political power, but this is largely as a response to the failure to manage Earth properly and not a real scientific endeavor. No one ever discovers any original Mars ecology.
I don't think there's ever any evidence of contemporary native Martian life. There is a line in Blue Mars that says:
Scientists working for the metanats had concentrated on resource exploration and evaluation, and had found signs of ancient oceans, of the early warm wet atmosphere, possibly even of ancient life.