Mikehotel152,
My two cents worth. Firstly, I reckon Skyjob and vapilot20014 are onto it. Fuel is being pumped from the R tank and into the Centre tank through the centre tank r/h boost pump due to a failed or partially failed boost pump outlet check valve. Crossfeed valve position (open/closed) is irrelevant due to the valve being mounted between the centre tank boost pumps in the engine fuel feed manifold, so even if it was open, fuel would still flow through the r/h pump and into the centre tank.
You did not mention anything about your first two sectors, so I can only assume that you arrived with <4tons of fuel both times and zero in the centre tank. You have to have <2tons in the l/h tank in order for centre tank fuel scavenge to operate.
You then tanker fuel and arrive with 4tons and 60kgs in the centre. So with 4tons in the wings no scavenge occurred and hence 60kgs in the centre. As Skyjob pointed out, very low fuel levels in the centre can cause erroneous indications. That 60kgs could have been 100kgs for all we know.
Next sector depart with 6tons and approx 4tons at TOC so scavenge pump kicks in and at next port we have a fuel imbalance but no fuel in the centre.
Mikehotel152 and its easy,
I am very surprised that your respective engineers did not carry out a stick check of the wing tanks to confirm ACTUAL tank quantities compared to indicated.
Lightning5,
Close but no banana. Fuel will drain into whatever tank that you are refuelling at the time when you finish refuelling.
Hope this helps.