OK... short version of how the cooling system works. You have your closed cooling system with the radiator and the engine. Attached to the top of the radiator is a hose that leads to the overflow tank. Since coolant expands and contracts as it respectively heats and cools it requires that overflow to expel and intake coolant as it needs to. A drain tube comes out of the overflow tank in case the whole system has too much liquid and will drip coolant if it needs to.
Your overflow tank has hot and cold marks on it. Do your levels exceed the hot mark? If so, then it's getting rid of fluid that it doesn't need.
If you are losing coolant from a different tube, then that means it's a different problem. Trace that line and let us know what it leads to.