This is a big no-no. Always store the extra propane tank, whether for your camp stove or grill, outdoors. Propane tanks are not designed for storage in closed areas. If the valve leaks, propane gas can seep into the garage, creating both an asphyxiation risk and a fire hazard. Once propane fills the garage, a mere spark or a pilot light can ignite the gas and send your house up in smoke.