Our 321s 'hide' about 200kg of fuel during the cruise, but our 320s don't. The 321s have jet centre tank pumps, the 320s have electric centre tank pumps - I don't know if this is the reason for the hiding behaviour?
I have also noticed that the FQI system can give an incorrect total value when tank contents are transferring. For example when the outer wing tank valves open and transfer their contents to the inner tanks.
I royally cocked up a fuel leak in the SIM by asking CC to go and view the source of the leak. Wasted a lot of time and fuel - and I did not get a clear answer. Best course of action seems to be to ignore that first part of the QRH drill and jump to the part of the drill that assumes the source of the leak is not determined?