Small leaks inthe plumbing can easily escape notice and be almost impossible to detect. Also do you have a "priming port"? If so how is it capped off? If you just have a plug or cap on it then that would be the first place I would look. If this does turn out to be the problem, then romove the cap/plug and replace it with a
ball valve that will close and form an air tight seal. I had the problem with my first CLS and when I turned it off and added the valve it gave the water the change to seep from a joint that had a very small leeak that was not detectable when running. Good luck.
Robert