As much as I want to believe that there is life on Mars...what if:

Deep water seas existed early on Mars.
Thick atmosphere including Methane and Ethane existed.
Condensation and marine deposition of Methane and other Hydrates occurred.
Subsequent gradual thinning of ocean and atmosphere.
Deposition of continental sediments.
Deposition of evaporites.
Slow migration of water vapor and hydrates from geologic structural traps into the near surface permafrost zone.
Billions of years of equilibrium with the dwindling atmosphere till only the thicker sedimentary basin patches of permafrost remain.

All those theories about how Methane has to be biologically produced seem to overlook the presence of sedimentary layers on Mars that could provide structural traps for atmospherically derived hydrocarbons over geologic time scales. The permafrost layer itself would form a trap in many cases. Are the 3 regions cited, Arabia Terra, Elysium Planum and Arcadia-Memnonia, sedimentary basins? If the distribution of Ethane matches that of Methane, the case for active bacteria is weaker.