Seems to me if there was an outright leak of some kind you'd either have a puddle of beer or an empty CO2 tank.
So, what kind of beer lines are being used? Straight vinyl, or barrier-lined? I'm not certain that straight vinyl is permeable, but I use barrier-lined and don't ever recall the first couple of ounces of beer being flat...
Ball lock connectors have a spring and valve within which can retain pressure when the tap leaks. Try over pressuring a keg and turn the gas pressure down and you will see this in action on the gas side.