The toilet tank may have a silent leak. Drop a little food coloring in the tank, wait 10 minutes without flushing, and see if the color appears in the bowl. If you do see color in the bowl, then you have a leak.
Check for moist spots around indoor plumbing fixtures, under the house, and around outdoor plumbing.
Replace worn washers in faucets and shower heads. A little drip wastes many gallons each day.