leaking rusty water

You may have an internal leak in the engine, from a headgasket or crack in the head or block. You will generally see white steam from the exhaust if your losing a lot of coolant, but small amounts may not show up. Sometimes an external leak can be hard to find because it may only leak at certain temperatures, and it may only leak while driving and it may not leave any sign of leakage. I would have the coolant system checked for hydrocarbons, this would indicate an internal leak.