1999 V70T5M. The coolant level in the surge tank drops over the course of a few days, even when the car is parked. So far, I replaced the radiator, which showed leakage between the core & tank when pressurized, and the surge tank, which showed a hairline crack on the bottom. When pressurized (cold) the pressure will drop from 18 psi to 17 psi in about 3 minutes. There is no dampness at any hose connection.

The engine does not run hot, overheat, or smoke. There is no dampness under the carpets, but there is intermittent smoke from the right side center dash vent when the A/C is cooling. I do not know how to check for leakage within the turbo (if such leakage is even possible).

Right now, I'm inclined to think heater core, but I do not know where the leaked coolant is going, since it doesn't seem to be going onto the floor. Is there some way to check for leakage inside the heater box, short of tearing it completely apart?