So did Mariner 6 -- for a few weeks, at least
The scientist in charge of the infraed spectrometer on Mariner 6 held a press converence after the flyby to excitedly announce that he had detected methane in the atmosphere, near the south pole, specifically.
It was soon discovered that some spectral features of solid CO2 were a better match to the spectral data -- dry ice, not Martian cow belches.