If it is a teeny tiny little leak, a normal charge may not leak enough to detect. If you have a really sensitive sniffer, your luck increases. By stuffing in non-condensible gas, the pressure can be raised so the gas leaks out faster. Once in a while, the seal will fail when the pressure is raised, so having something benign inside is a definite plus. Usually raising the pressure close to the labeled design will reveal the problem.
Also, having non condensing gas in the system will tell you how bad your leak really is. Charge to a target psi, and the gauge pressure will decrease steadily as gas leaks out. With pure Freon inside, the condensed gas will boil off and maintain gauge pressure.
Of course, a good sniffer wand or thick bubble soap solutions will increase your luck. But once you find the source of the leak, then what? The gas has to come out anyway.
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