The methane observed on Mars was so small that it was below the detection threshold of Viking's in situ measurements. The best on-line comments on Martian methane is:

http://mainlymartian.blogs.com/

The same website references a paper that calculates the total quantity of Martian methane could be produced by 2000 cows. That's a lot of cows but remember this is for a planet with the same surface area as the Earth's continential surface. Such a tiny amount of methane could easily be produced by a non-biological process. The only way to be sure the methane is of biological origin is to sample the methane in situ and determine the methane's carbon isotopes. Until that measurement has been made it's all idle speculation.

Gary