Well, turns out it is the radiator that has gone bad. The radiator is made of both plastic and metal, and the seam where the two materials are bonded together has deteriorated and come apart. Not as expensive of a repair as I thought it may be - but still isn't cheap.

Thanks! I'll do that and report back

Hello, I have a 2001 S60 T5 with the 2.3L. The car started leaking coolant from looks like the reservoir overflow tank. It isn't a constant leak, and it seems to happen after the car is driven, then parked. I have replaced the thermostat, but that was not successful. One thing I have noticed is that...