It is probably the toilet valve. Purchase a good toilet chemical that has the properties of lubricating the valves, cleans the sensors as well as break down of waste. When you dump, pour some in your toilet and let it sit. This is so it will penetrate the rubber gasket and allow the valve to slide on it, fully closing. It the valve in the toilet does not close ALL the way the water valve on the side of the toilet won't close. Also, try some silicone on the spring after you clean it so cut down friction. If you are worried about the toilet overflowing, purchase an alarm available at Home Depot for about $9.00. You just set it on the floor and if there is water it emits a high frequency tone.
