I think Trail Rat is right about the fill loop:-
Aren't fill loops a removable key or flexi hose and ALWAYS had to be disconnected after fill??? I thought WRAS reg was there deliberately so there was no chance nasty heating water could back fill the drinking supply? Also, the mains water is higher pressure than the heating circuit, so how could it ever leak that way?
My money is on pressure relief valve, possibly but not always related to pressure vessel failure (mine weeped just by using it to remove pressure to work on a rad without ever "going off" due to over pressure). Or a small leak somewhere else - it must only be a weep somewhere as it obviously holds pressure when they refill the thing. Not a lot of water loss is required to drop the pressure (just the amount used to move the diaphragm in the pressure vessel plus compress any air in the system).