The lack of methane detection at the Mars Science Lab site is no surprise, since the emission of methane may be highly localized to a few locations on Mars. That's the issue with so few rovers - you only have a few data points to work with regarding the detection and/or non-detection of methane. Ground-based data does seem to indicate the presence of methane (10-40 ppb) at several localized regions on Mars. Also, you have to rely on the assumption that the instrument on board the Curiosity rover is working correctly, and is calibrated correctly for the detection of methane (CH4).
Hungry...for more data!
It's incredible how localized some geological phenomena can be. I work with rocks that are over 10 km thick, but the rocks that I need to do my micropaleontology work are only in layers that are 1-5 mm thick, meaning that my work is literally looking for rocks that are 1-in-a-million. I'm pretty sure many other geologic datasets require similarly narrow specifications that involve the right combination of formation, deposition and alteration conditions. Mars may even be more restrictive, given its start-stop geological history and ancient weathering.