2004 s60 AWD. Have to fill the cooling reservoir every week. System maintains pressure. No apparent leaks. Mechanic replaced thermostat housing and thermostat. No one seems to know where its leaking. No water or moisture inside the cabin. No white exhaust. I do need to replace the low level sender unit as it intermittently reports low coolant levels.

Any suggestions?

 

did the mechanic do a pressure test? Look carefully around the upper and lower radiator hoses and side seams of the radiator for any discoloration. I had something similar on my son's S40 and it turned out to be the upper radiator hose - but I really had to look to see the stains.