Water loss SL 320 R6 R129

Hello! Have a SL 320 R 6 BJ 96 with not even 47500 km. Now I had to wait a few days with the car with running engine 8 minutes before the gas station. Since then always the yellow lamp goes on that lacks cooling water. I then refilled the cooling water level and after a short ride again about 25% of the cooling water content is missing. I then refilled again and now the same amount is missing again, where it will then no longer be less. What can be the reason for this and what to do? Thank you for your help. Best wishes for the day The Australian