If the truck has past the Belgian border , then it wouldn't surprise me it was on the way to Eastern Europe , or further, Russia , Ukraine, ....
Remembers me of a similar case , by which an Armoured car , M 8 or M 20 has been stolen in the Ardennes, anyway to make a long story short , it has been detected by very alert border control people , just before crossing Ukrainian border . It was transported in a conventional lorry , manned by several drivers , to do the ride without any loss of time.
These are very organised people , all has been well planned , and once it has passed the border of former named country, it will be very difficult to recover