Wild_Bill wrote:

Previously freezing, which could've expanded the tubing's OD is a likely
possibility if the climate is seasonal.
The tubing might then have micro-cracks, which I've found before.. the
resulting leakage can produce a fine mist that's difficult to see, and may
only be detectable by feeling the mist on sensitive skin, such as the
underside of a forearm.

Would some aggressive acid flux and a good slathering of silver-bearing
solder seal the microcracks?

Thanks,
Rich