Volvo: Hidden water leak?, cheap petrol, valve stems

Question
My B200F engine is losing water steadily at about half a cupfull per day, with no sign where it is coming out.  I did have a leak on the heater valve, but I have replaced that.  The head gasket has been changed after I had a problem recently with sticky valves, now cured which I have attributed to some duff cheap petrol.  There are no signs of pink coolant stains apart from under the former heater valve.  No sign of leakage from water pump shaft seal.  The oil is brown/black - no creamy gunge.  Engine has done about 170,000 miles.  Compressions are all good, and valve stems when measured were all within new tolerances.  Any ideas - I have run out.

Answer
John,
Heater core. Could be if the car is an automatic that the trans cooler has a crack and sucking coolant into the trans. Sucking some coolant into the engine and it is being burned. Small leak around the block and the coolant is burning away and not making it to the ground.