Purging tanks
I have no experience on 747s but on most large aircraft I have worked on if a tank is/has vented you can use the tank booster pump to transfer some fuel into another tank, lowering the level away from the vent line in that tank. I would imagine the tank would be quite full for that type of sector.
Was it warm at your departure airfield? Most fuel is stored in underground tanks and is thus quite cold. Refuelling into a warm airframe can cause the fuel to expand into an already full tank possibly causing a vent. As has been said before, not unusual. You will also be surprised what just a couple of kilograms of fuel looks like on the ramp after capillary action spreads it about!!!!