I figured it out the other day. I took another look at the heater hoses and there was wet coolant residue at the heater core ends of the hoses. The clamps weren't super tight, so i tightened them down (I replaced the heater hoses and used new clamps 5 years ago). I figured I was pushing on the heater hoses when accessing the transmission dipstick when I was checking the ATF level, and the loose clamps let some coolant leak out when the hoses were pushed on. I ran the car and even pushed on the heater hoses, but the connections to the heater core are still dry.