My natural gas usage has increase significantly in the last 3 months. This home is rarely used (10 days a month).
Is it possible that the 10 year old natural gas water heater has become increasingly less efficient?

My gas usage went up when there was a water leak in the hot water line under the house.
Efficiency probably is down, but should not be that noticeable I would think.
Turn the thermostat way down when not in use and it won't take long when back in the house after turning it up, before there is hot water. Maybe an hour or two for a bath or shower...

My natural gas usage has increase significantly in the last 3 months. This home is rarely used (10 days a month).
Is it possible that the 10 year old natural gas water heater has become increasingly less efficient?
Personally, I would cap off all gas fired appliances and the gas meter and then put a thirty pound gauge on it and pump it up to 15psi to see if you have any leaks in the system.

If the gauge goes down, pump it up again and then start spraying the fittings with a mixture of liquid dish detergent and water.

If any of the fittings start bubbling, then you have a leak.