Came back from a short 15 minute drive yesterday and noticed that my car was steaming from the engine, so i opened the bonnet up and checked the water and there was barely any in the resevoir, i topped that up and thought nothing of it i then used the car again today to find that it had no water in the resevoir and that my engine now has rust coloured water stains all over, so just wondered what mught be the cause and how bad it is?