Theory of what it is.
Check this theory. The duct is from the outside air vent (at back of hood).
I had the air mix control (hot/cold for heater and A/C) set all the way to cold, so damper should have been full closed. Heater Core blew, pressuring up the chamber where the outside air and heater core air mix. Pressure may have sealed damper even tighter (don't know, kinda all depends on which way it swings). Pressure had to go somewhere, so it blew out around the outside air duct.
Least that's what I'm hoping, since it's only a heater core and not a head gasket. Just luck that I didn't get sprayed with antifreeze or at least dripped upon.