There are many reasons why your toilet regularly refills its tank because of leaks from the tank into the bowl or worse, to the floor. But this leak is so uncommon it is often missed.
Apologies for occasionally referring to the toilet BOWL and toilet TANK incorrectly.
Last but not least, the mechanism for filling a toilet tank, called a "fill valve" or "ballcock," which is controlled by the float that rises and falls with the water level did NOT have to be removed to perform this repair. I chose to remove it so that I could do a thorough cleaning and inspection of the tank. In most cases, folks will choose to leave the fill valve attached to the tank and only disconnect it from the water line so that the tank can be removed.