The Beagle-2 Mars lander has an experiment onboard to try to 'sniff out' methane
in the martian atmosphere near the surface.
Apparently the thinking is that if Beagle-2 detects a 'whiff' of methane,
that will be compelling evidence for the contemporary existence of
biological life beneath the martian surface...Or at least from the decaying
organic detritus thereof.

I disagree.

I could think of at least one method of generating methane by a non-biological,
non-artificial method. It would involve an asteroid/meteorite impact and shock
heated carbon-rich & hydrogen-rich material within or deep beneath the impact area.
Would you agree with me on this, Nick?

Jacob