As you say, "I'd be far more likely to believe that source is related to volcanism, maybe slow outgassing from magma bodies somewhere" but how could anyone distinguish between this scenario and subterranean colonies of methanogenic microbes? For instance, in the Lidy Hot Springs, Idaho, at depths of 200m or so, the US Geological Survey found communities of approx 99% archaean microorganisms that convert hydrogen and CO2 to methane and live in the near absence of organic carbon (Nature 415, 312-315). Is there any way to "look" at methane on another planet and definitively say, yes this is biological, or yep, its volcanic?