I know it's (extremely) unlikely there is (& perhaps ever was) any life on Mars but having had a glance through the missions I can find I can't help thinking we've not really looked in the best places for it yet.
If I wanted to look for possible life on Mars I'd want to go to the equator (as it's warmest & gets most sunlight) & look for the lowest places I could find there for their slightly higher atmospheric pressure.
There's two spots that jump out at me as worth a look using those criteria.
A crater (I can't find the name for) at approximately 0°N 165°W
Where the Valles Marineris flows into the Northern hemisphere at approximately 0°N 35°W
You can find them both on this Topography map of Mars.
So, have there been missions to (or are any planned for) any of those?