I was once called out on a job like that. HO couldn't figure out why the bill was so high. I Checked all around and while I was in basement, I heard something running. Long story short, her well developed a leak in it. The water kept running from the bottom of the well to the top of the well, while never leaving the well casing and the well motor was running for a few days constantly.
Also ask HO if the bought a new self cleaning oven or refrig. there are heaters on the frig to de-ice them. Another point would be if they have elec. water heater. Did they change the temp of the water? Good Luck.
