Obviously there is something affected by the heat. This could be from expansion and contraction of the flooring, or it could be condensation, or it could be a leaky gasket on the tank.
Your's is a tough diagnosis. A real important fact is what side the leak is on. If its on the right side of the toilet, its probably not the water line. Does the tank move easily when you push it back and forth? I would check the bolts in the tank. See what age they are. It may be some stress is placed on the tank as the water line pulls on the tank when the floor is heated.
If its condensate, you should be able to insulate the inside of the toilet tank with bead foam. 1/4 inch thickness glued to the inside of the tank.
Or it could be a fantom.
Keep a watch on it.