Wondering if it wasn’t an attempted insurance fraud. Ok the furnace could have a problem. The size of the problem would be a huge one, easily detected and gas cut off and locked.The leak wouldn’t (shouldn’t) occur in a gas furnace with a proper functioning thermocouple. If the thermocouple wasn’t functioning properly, gas flow should stop. If gas flow continued unchecked, pilot (electric) wouldn’t activate because of the thermocouple. It doesn’t add up. Luckily, the resident was away from home a few days. Why leave the unit on if you’re away?
You leave a furnace on when you are away to prevent pipes from freezing.