Why water in my car reserve tank goes out when you stop the engine?

The water reserve tank in your car holds extra coolant for the engine cooling system. When the engine is running, the water pump circulates coolant through the engine block and cylinder heads to remove heat. Some of this heat is transferred to the radiator where it is dissipated into the atmosphere. If the coolant level in the reserve tank is too low, the engine overheating warning light will come on. Once the water pump stops circulation the system cools causing a vacuum hence drawing the water from the reserve to bring its levels in accordance with that in the system due to pressure changes with cooling. That why you find it empty