cvx - Interesting theory. I was just thinking that if this were engine oil I was inspecting, I would conclude I had a blown head gasket, allowing coolant to mix with the oil. The liquid that was coming out of the fill hole was very bubbly, more brown in color that ATF, and also quite thin and watery, so you may be on to something.
I checked the coolant level, and it's very low (surprised the car isn't running hot), which supports your theory. So, if I understand correctly, there's a heat exchanger that allows the engine coolant to heat up the auto trans fluid when the car is first started, and once the auto trans fluid hits the target temperature, the thermostat, located in the bottom of the coolant reservoir, shuts off the flow of coolant. So, you're saying I might have a situation in my transmission heat exchanger which is allowing coolant to get into the trans, which would account for the frothy liquid coming out of the trans fill hole.
I looked on realoem, and couldn't find a picture of the transmission heat exchanger. Do you know where it is? Any idea by what mechanism a broken transmission heat exchanger could allow coolant to get into the transmission? Thanks.