I am having a problem with a toilet tank draining down. There is no signs of leakage on the floor. I have replaced the flapper and the problem continues. The fill valve is a Fluidmaster (I'm not sure what model). Any ideas of what might me happening? Could it be siphoning off? What are any suggested fixes?
Thanks in advance.

First of all let's nail down where the water's going. Tonight place some food coloring in the tank. Next morning check the bowl. Any color down there? Let me know. Tom