Service Valve Leaks
Suspect the Service Valves.
If you have done everything possible to locate a refrigerant leak and you still can't find it, don't overlook the service valve.
I've been "burned" by service valve leaks so many times that I always check the service valve for leaks.
Sometimes the Schrader insert is not screwed into the valve all the way. Sometimes it just doesn't hold pressure very well. Sometimes, putting the dust cap on opens the Service Valve insert a little bit.
Hooking up your manifold to the service valves conceals any leaks in those valves. Your pressure tests and leak checks will show a 100% OK , but as soon as you remove your gauges, the service valve will leak again.
After you remove your gauge hoses, put everything back together and tighten it up. Put the dust caps on. THEN, check the service valve for leaks.