Very likely, and there is a five-minute test for it. Take the lid off the tank, drop in a few drops of food coloring, and leave it alone for twenty minutes without flushing. Come back and look in the bowl. Color in the bowl means water is passing from the tank through the flapper, the rubber seal at the bottom, and running continuously into the drain.

A silent leak like this passes hundreds of gallons a day unnoticed. It is the single most common cause of a mystery bill increase I see in residential plumbing in Sacramento, and it is also among the cheapest to correct. A flapper is an inexpensive part sold at any hardware store, and swapping one takes a few minutes with the supply valve closed.

Run the test on every toilet in the house, not just the one you suspect. Households find two at once more often than you would expect, which is why toilet repair in Sacramento so often turns into two small jobs on one visit.