Hi Scott!

What Klaus said and yes, the air guide must be removed to really get a good look at and access the radiator drain; 7-8 Torx screws (my reference is a pre-facelift P1 S40, so YMMV).

The other possibility to investigate is leaking at the heater core. As you may know, the hot engine coolant circulates through the heater core at all times, regardless of the climate control setting. There are two o-rings at the connection between the heater core and the fitting at the firewall. Eventually after so many heat cycles the rubber rings will give up and begin to leak. If you can get your head down in there to see them, it should be plain to see if they have been slowly leaking fluid.

Please whatever you do don't open that expansion tank unless the car is good and cool. While back a coworker burned himself rather badly when opening the system on his Chevy Cruze (bad head gasket).

-Will
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XC60 / Odyssey