1998 Subaru Legacy, 2.2L, 147,000 miles

I see some coolant on top of the engine.  If the car seats for a week
and then runs, white, sweet-smelling smoke comes out from the engine
bay area.   It looks like coolant drips into the Y-pipe and burns.

However, no smoke from the tailpipe so the leak appears to be
external. It's minimal and in day to day driving not obvious.

The engine does not overheat nor does the temp fluctuate. The heat is
slightly on the warm side (versus hot) but does get hot eventually,
after 20 mins of driving.

I know that Subarus of that vintage are well known for head gasket
issues, but I thought that 2.2L was fairly bulletproof.

I looked at the radiator hoses but cannot find the source of the leak.
It's on the passenger's side of the engine.

http://allwheeldriveauto.com/subaru-head-gasket-problems-explained/