Only two places I can think of this happening is either the oil/coolant heat exchanger or the primary oil pump in the rear casing.
For the heat exchanger It could be an internal leak and oil pressure is stronger than the max 13 psi of the coolant system.
For the rear casing oil pump look for the weap hole near the thermostat housing for oil.
Where the main oil pump shaft comes out of the casing and screws onto the water impeller there are a few seal's that could have failed and it could be oil pump pressure that is leaking past it into the coolant side.
Can't think of anywhere else the oil and coolant are close enough to get mixed.
General thoughts on an experiment: What about removing the coolant lines going to the oil cooler and looping it back to itself and then drain the cooler and pressure testing it.
Or run the engine briefly to see if any oil starts coming out of the coolant side. I don't like this idea though.
