I rechecked the right fuel tank and the good news is that it is ONLY leaking from the BNC connector, the rest of the tank outer skin was bone dry after 24 hours full of water. I did some brief research on this and it seems to be a common issue with BNC connectors. I had hoped using top branded Amphenol connectors would prevent this issue but it seems not.
So, how to fix it? I drained the tank down completely and have left it upside down to dry thoroughly. Once it is dry dry I will reinspect the sealant around the connector. One forum I read suggested it might even be flowing from the other end of the wire in the tank. This is known as wicking. I can’t quite believe fluid could flow all that way but I will check the sealant at all the connections inside the tank as well as at the BNC itself.
Given my now detailed knowledge of water testing the fuel tanks I decided to retest the left tank. So I quickly filled the tank with tap water, dried all areas including the tank attach bracket and left a tissue under the BNC connector to capture any leaks.