Is there a safety warning about hot water systems that haven't been used for two weeks?

Yes — under certain conditions, hydrogen gas may accumulate in a hot water system that has not been used for two weeks or more, and hydrogen gas is explosive. Before using the dishwasher after such a period, turn on all hot water faucets and let the water flow from each for several minutes to release any accumulated gas. Do not smoke or use an open flame during this process, as the gas is flammable. (Page 2)