loosing coolant

hi all, i have a gu zd30 and the coolant dissaper at a rate like 2 cups every 100km, this happen for the last 12 month. i have change the radiator , radiator and heater hosses, replaced heater gaskets, the water pump is ok not leaking, replace the radiator bottles and both caps ( radiator and bottle) sometimes i can feel steam in the cabin, its not the heater as i have bypass it. check the engine compression its ok the same in all cillinders, no water in the oil...i dont know what else to check....i dont think are the welch plugs as i dont have an water leak that is visible....any ideas, anyone?

I had the same problem in another vehicle. I suspect it was leaking out the exhaust from inside the head which happens occasionally. I never fixed it.