My guess would be the plastic pipe under the intake manifold. It is known to get brittle with heat cycles and eventually crack and fail. If you don't know what I'm talking about, search for "mess under the intake." You might as well delete the TB heater like in the thread and replace all of the old vacuum hoses, being in Texas and all. Do you have any leakage in the passenger foot well, carpet damp? That tends to be the most common symptom of a leaky heater core / hoses. It could be a few things, but pulling the intake manifold and checking that plastic pipe is a good start, because if it completely fails, it wouldn't take long for things to go south, fast.
Edit: I missed your last bit about it being a tiny amount. Are you sure that it is coolant and not just condensation from the AC system?