If you hear the Fluidmaster cycle and the water level in your tank doesn't continue to rise to the top of the overflow tube, it is almost certain that your tank is losing water. This is most likely through the flapper.
One sure way to test this is to put a few drops of regular old food coloring in the tank, let it sit for a few minutes and see if the water in the bowl is colored. If so, replace the flapper.
Beware, some toilets only work right with O.E.M. flappers. Home Depot and Lowe's sell several different flappers. Try to find one that lists your brand toilet specifically.
If you want to save water there is a great alternative to a Fluidmaster that not only helps to conserve water but it actually helps keep tank clean too. It's called a Hydroclean and you can check it out here:
They sell these at Home Depot for about $13. I put one of these in one of my toilets and it really works as claimed.