Well it wouldn't suprise me if they did find methane, considering it's just CH<SUB>4</SUB> (Carbon and Hydrogen), considering the martian atmosphere is CO/CO<SUB>2</SUB> and the major preportion of the universe is suppose to be made up of Hydrogen and Carbon (Along with the other base elements like Oxygen).
However I couldn't suggest that the CH<SUB>4</SUB> content would be in quantity, considering that if it was, Mars would have a higher water table than it apparently has. (Consider the chemical components altering state similar to how our atmosphere does. Sometimes we have higher concentrations of CO to CO2, which is also effected by Evaporation and climatic changes based upon the earths tilt at given times of the year.)