The original Viking landers found evidence of life. It was discounted as strange Martian chemistry and not real signs of life but the basis upon which it was discounted has been largely dis-proven.
NASA keeps finding more evidence pointing to the existence of microbial life, methane gas, that is localized in origin and comes and goes. The only real options for methane are life or geological processes and Mars appears to be geologically dead, no evidence of currently active volcanoes, no significant magnetic field, which leaves… life.
Recently, Curiosity discovered nitrogen, in the form of nitrates, another key element in life. NASA is quick to point out that they don’t think the nitrates were created by life, probably the result of past meteoric impacts or lightning.
Curiosity also found chlorobenzene and dichloroalkanes which are chlorinated hydrocarbons, the product of chlorine found on Mars and Martian hydrocarbons. As with Methane there are possible geological processes that could create these hydrocarbons, but again, Mars is geologically dead.
Some of the deeper canyons on Mars have sufficient atmospheric pressure that liquid water may be possible.
So when do we get to look for actual life? Or when are they going to announce they’ve already found life?