I've just been through this in my Z (not a Subie obviously and no turbo either, but it might still be relevant). I noticed my car overheating months ago, found it was quite low on coolant and topped it off. The oil was clean as could be and it there were absolutely no puddles under the vehicle or traces of coolant in the engine bay. I haven't been driving the Z much so it gets parked for a couple of weeks at a time during the winter. I have dry-rotted weatherstripping, so I assumed that if I had a wet floorboard it was from snow / rain water seeping in. A couple of weeks ago, after a nice weather drive I noticed that the driver's side carpet was dry, but the passenger side seemed to be completely soaked. Figuring it was rain water I pulled the carpet out of that side to find a green puddle in the passenger footwell.
Gee, found where my coolant was going. I've already got a replacement heater core ready to go in.