Were you able to see what the plume smelled like? Turbo seals do fail. The turbo on my liberty uses a bushing rather than a ball bearing for the turbo shaft. They are known to fail, and sometimes catastrophically causing all the oil to be dumped into the intercooler.
A couple other things to look at. Is there any oil in the coolant reservoir? Subarus have head gasket issue, and the normal signs are an occasional slight rise on the temp gauge after a long hill climb, and oil floating in the top of the coolant reservoir. Another thing you could do is send your oil off for analysis. The should be able to tell you if there is any coolant in it.
