first of all, does the presence of methane necessitate the hypothesis that the source is biological?
If I recall correctly, there are satellites in our solar system with atmospheres that have a lot of methane in them. Certain moons of Jupiter or Saturn. I’d have to look it up.
Secondly, methane could be trapped in the upper crust due to a more distant past of biological activity.
I think I recall reading that at one time Mars had a much more significant atmosphere than it currently does and the climate may have been warmer.
“does the presence of methane necessitate the hypothesis that the source is biological”
No, methane is one of the very simple hydrocarbons that can form naturally.
There are still about 70 degrees in daytime near equator in summer.