I'm calling it a mystery leak because NOTHING shows any signs of coolant without pressure in the cooling system, so I'm more than happy to get hints. Had the same problem on the Volvo until I got a pressure tester on it and found a heater hose I didn't know was there, dribbling nicely.
Rad is completely dry as far as I can tell, at least on the back side and bottom.
Will keep looking.....
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!