A signal that is too weak for direct detection by the orbiters is a setback. But perhaps they might still be useful in searching for a kind of "fossil" evidence tracing the methane destruction chemistry.

Suppose a scenario where the venting has been using the same vents and that these vents have opening every year for the past hundred thousand years. As a field geologist your comments on the viability of this scenario are welcome. The idea is that over the most recent geological age that methane will be as quickly destroyed over the past 100,000 years as it is today by whatever mechanisms that are in play today. While a portion of the gas will temporarily escape into the local atmosphere, where the venting occurs is where the destruction could be the greatest. Byproducts of the destruction may tarnish the immediate area around the vents such that after each of the 100,000 years there could be a build-up of a chemical tracer that could point to a cone that could be detected, now at percent levels, where the center of the cone indicates a possible long term venting source. This kind of like a geyser build-up of chemical gunk from destruction products that have had time to collect to become dense enough to be seen by CRISM. I don't know if this is actually feasable.

For example, in one of the methane regions, there has been a localised confirmation of carbonates. Now I am not suggersting that the carbonate signal is somehow related to the methane destruction, it may just be a coincidence. But looking at the carbonate signal as a "rainfall" that built up over an extended period such that it can be detected by the orbiters is along the lines of "fossil" evidence build-up of methane destruction is symbolic of what I am suggesting.

So it might still be worth looking for this "fossil" evidence, assuming there is anything valid about the premise.