It would be inaccurate to say that Curiosity detected no methane. During the telecon, it was stated that Curiosity did see a difference in the methane lines between a chamber full of Mars air and a vacuum.
The issue was that there is noise in the measurement. Assuming a normal distribution of the noise, they stated that two standard deviations from the mean, i.e. a 95% confidence level, would spread out the measurement to be somewhere between 0 and 5 parts per billion. Since this range includes 0, they could not claim a confirmed detection of methane at a 95% level of confidence.
However, if we consider a range of only ONE standard deviation from the mean, we would still get a confidence level of 68%.
Suppose the mean they detected was 2.5 and the standard deviation was 2.5, which is consistent with what was reported. In that case, there would be a 68% probability that the true value for methane was in the range 1.25 to 3.75, which is above 0. In other words, the odds are better than even that there is methane at the Gale Crater site. Because of this, I am confident that they will eventually have a confirmed detection of methane when they refine their methods to lower the experimental error.