Cooling water loss

Hi, everybody! I have to refill about 1 liter of water every day at our Mini One 98PS. He probably does not lose this water by defective hoses or containers, but only in driving operation. I had it pulled off for several hours in the workshop. Loss: negative… At the moment, not the biggest problem, but when the temperatures continue to fall, it makes for a little fun every morning before the start of the journey to make the creepy refill act, but above all that the necessary Frost protection is missing. We fill in normal water each time. Now the question: Who knows this problem with this engine? Which gasket is there in the warm run that leaves so much water? Or is it not a gasket at all, but the motor/cylinder head as such (worst case)? Thanks in advance for all participation…