I think I may have figured out where the intermittent leak is.
Following your technique in Tanklets I got the heater to fire. The pressure in the system remained at 80psi and the T/P valve continued its slow drip pace. (I know I have a second leak besides the slow dripping T/P valve.) After a few moments I began to hear a high pitched sound coming from the top of the heater, under the outer sheet metal, that sounded like pressurized water escaping from a pin hole leak. This sound continued until the heater shut off at which point it too stopped.
It seems to me that either the hot or cold nipple is leaking at the point where it is threaded into the tank. Based on what you have said, and where the bulk of the water collects I think it is the cold. That nipple is somewhat rusted at the top of the tank so I am concerned that removing it would be difficult. I am also worried that it might break off in process of removing it.
What are your thoughts or recommendations?