I would remove the flush valve that holds the flapper, and sand the inside of the tank.
If the inside of the tank is rough, or if a small bit of clay is under the rubber seal, it can leak slowly.
I just installed a new Toto Dartmouth. I had this exact model in my old house, and wanted the same toilet in the new house.
The new toilet loses water from the tank over a period of 1-4 hours. The total volume of the loss is @ 16-24 ounces. This is causing the toilet to begin to run when the water level goes below the water line.
This has happened on two tanks, and three flappers. The plumbing supply that I purchased the toilet from is not very friendly, and seems to want to go out of their way to give me a hard time.
I called TOTO and they are at a loss as to why this keeps happening. I have installed these toilets before, and have not had a problem. This is the fifth TOTO I have installed, and the second Dartmouth model I have installed in the past 3 years.
Try manually pushing down on the flapper ball to see if it seals and stops leaking. If it does stop leaking, try installing a different brand of flapper.