The conclusions are based upon spectra with lots of background noise
The quantities of supposed methane on Mars are tiny and the conclusions are based upon spectral data. Upper atmospheres through photo-dissociation and recombination can produce bizarre molecular species, e.g. C2O. One sees this sort of molecule in the shock layers of spacecraft entering a carbon dioxide atmosphere. As far as I know, the spectra of C2O is not tabulated. How do the people claiming they have seen trace quatities of methane know that they are not observing C2O or something even more bizarre?
Gary