Somehow, somewhere, water is escaping from the water tank.
My son, the engineer (that's scary), helped to replace the rubber gasket between tank and commode as well as the bolts and rubber washers and the flapper. We cleaned, or thought we did, all around the connection between tank and toilet. The problem persists and the tank refills about every 40 minutes or less. and YES, I keep the water turned off. Give me some other ideas. The float is one that slides up and down the stem.