Interesting development. I happened to open the fridge my King Keg is stored in. When I closed it again I heard a low whining noise, presumably coming from the keg. Repeated a few times and yep, sounds like a leak in the keg
In other news I think I've found a way to test a keg for leaks. Presumably the pressure of closing the fridge forced the co2 out?
PTFE tape on all fittings. Filed down the rim. Filled with a gallon of water and turned upside down, water right out the pressure valve. The tiny clear plastic sleeve on the underside had slipped down beyond the tiny hole. Slid it back up and everything seems fine now.