I havd had two similar experiences to Chrome's.
First one was in the cruise to Tallinn. It appeared that all of the fuel had disappeared from the LH Outer tank, and no it had not transferred to the Inner. No leak from the LH Inner so flight continued to destination as the remaining was sufficient if a little skinny. During the descent the fuel magically re-appeared. Fuel sensor problem in the tank.
Second was very similar, bizarrely also during the cruise on a flight to Tallinn. It appeared that we had a significant fuel loss from the LH Inner this time. Applied the procedure to check Fuel uplift vs burn and fuel remaining etc. Fuel could not be accounted for, subsequent diversion ensued followed by engineering inspection. Ground engineers pumped all of the fuel on board to the RH wing tanks. Magically, all the fuel re-appeared. There was no fuel leak. Once again, fuel quantity (more correctly I believe, Mass sensors) in the LH Inner wing tank were faulty.