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I think it's a matter of suppressing expectations -- if no microbial life is discovered by direct sampling (or rather, rediscovered), then a lack of methane won't be surprising; whereas, if there's loads of methane, and no microbes, big problem.

The Viking Labeled Release Experiment and Life on Mars
by Gilbert V. Levin
Many hypotheses have been advanced and tested in attempts to account for the well-characterized activity detected in the surface material of Mars by the LR experiment. As shown above, these hypotheses have themselves been found wanting. The demonstrated success of the LR and the exquisite sensitivity with which it has detected microorganisms during its extensive test program with its record of no false positives can no longer be denied. No non-biological approach published, or known to the author, has duplicated the LR Mars data. Some laboratory experiments have produced positive responses, but the detailed thermal sensitivity exhibited by the variety of controls conducted on Mars has remained elusive in all such tests compatible with martian conditions. On the other hand, a combination of known properties of microorganisms, perhaps even those possessed by single species, could reproduce all aspects of the LR data. The biological interpretation of the Mars LR results is left standing alone. Recent discoveries of life forms thriving in extraordinarily severe environments on Earth strongly indicate that any alien organisms arriving on Mars might well and widely adapt to their new home. Application of the scientific principle leads to a conclusion: the Viking LR experiment detected living microorganisms in the soil of Mars.