Why haven't I seen this possibility brought up elsewhere?

Not that I think it doesn't have merit (it certainly does), nor have I
perused every scrap of literature on this subject, but something which
seems so relatlvely simple and is obviously a real event on this planet
makes me wonder why the possibility isn't being investigated more?

I know the idea of the methane coming from life or even lava on Mars
is an exciting one for many, but I worry that we will have a repeat of
Viking, where if we investigate the methane and it turns out to be some
"basic" geological process that our exploration funding for Mars and
elsewhere will dry up for another few decades.

We have hoped for and assumed there is life on Mars for so long that it
has taken away from exploring other interesting aspects of the Red
Planet. It would obviously be very interesting if the methane came from
a subsurface ecosystem, but every assumption we have previously had
about Martian life has turned out to be wrong so far because of our
past hopes and assumptions for that planet. The odds that the origin
of the methane is also inorganic is just as likely as not.

Of course it is possible that there is some life below ground on Mars
living off the planet's interior heat and the ice that we know exists in
certain locations. Microbes can certainly survive for eons in harsh
conditions. And I certainly want to see us keep exploring Mars, but
this perpetual focus on native life could ultimately permanently damage
further missions to the Red Planet. While at the same time it might also
bode well for human colonies (we won't be contaminating any Martian
ecosystems if the planet is dead), but the first human missions to the
fourth world from Sol probably won't happen until the 2040s and maybe
even later now.

If the taxpayers become bored with a Mars that has no life, there could
become a huge gap in our exploration and further knowledge about that
world - it has happened before.

And for my money, if we are heck bent on finding alien life in our
celestial neighborhood, we should be focusing on Europa instead.
There may be more than just microbes in that alien ocean. We
might not even have to dig through all that ice to find it, either.
Freeman Dyson even suggested that Europan remains might be
floating in Jovian space from comet and planetoid impacts with that
moon.