All it needs is a way to siphon off a small volume as the main load of fuel is transferred, and a hidden tank in which to store it so it can be taken for subsequent "re-sale".
Gentleman be aware when refueling in ALA. There is one fueler usually driving the same modified fueltruck. Part of the fuel is siphoned back into the truck. This regularly leads to a difference of around 500kg during each refueling.
Someday one of our engineers found out and informed all station personell involved around our aircraft. Since then, this special fueler and truck is not allowed to refuel our aircraft.
Thinking about notifiying the officials about this? Probably a good idea, but sometimes things work differently in those parts of the world. I think our company officials even did that - to no avail. In all likelyhood it's a bigger scam going on around there, so just prohibiting that guy and truck to refuel the aircraft is the easier way...
So, watch out in ALA.