Methane is lighter than CO2. You might not necessaril;y find much at ground level if little has been produced lately. If it's going to be found in quantity it will be at higher elevations.

With almost no oxygen, life there would be anerobic so theoreticaly there should be lots. The question is what about the seasons? Are we sure we are even in a Martian season that life thrives in, or is it lying mostly dormant waiting for conditions to improve? What I want to know is if it is winter or summer there in that spot right now. Winter goes to -225F. Summer is a nice 70F. I could see life up and running at 70F, dormant and just hanging on at -225.

Who knows what's possible or what might happen with a change of season.