Check water heater thermostat. Maybe it got turned up by a family member without telling him.
Check if the service feeds any neighbors’ houses. I’ve seen a few of those with family members living next door. One service won’t handle a new load so they feed from the one that can. People forget those details, a brother sells house, new owner uses stuff more & jacks the bill.
Check for hidden loads within the house. I did a job for a family on city water. Kept hearing a motor in the crawl space. They had been in a well before. Plumber had rerouted water years before but water pump was still connected and running. I killed the circuit but boxed it off where they could use it again if they wanted to, for a backup.
Check for any electric baseboard heaters covered by furniture. May be on & consuming. Space heaters in outbuildings?
Check for a range with bad thermostat, let’s oven element keep heating at low level. Not common but I’ve seen it once.
Attic fan set too low & never stops running? Stuck thermostat possible too.