The hot water system is showing unusual hot water consumption patterns that may indicate a substantial leak or extraordinary hot water usage.
To verify, wait for a period where no one will use water for 8 hours (e.g. overnight). Take a picture of the water meter dial at the start of the period, then another one 8 hours later. If the dial has moved, there could be a leak of that number of litres.
Please contact a licensed plumber for assistance.