Any ATC system which permits traffic not to be displayed in a sector merely because the computer does not know about it is a BAD ATC system. At Swanwick the only traffic which is foreground is known traffic. Aircraft on the wrong SSR code will be background. Thus if the code is to be changed on entry into UK airspace it will remain background until the code is changed. Only the tactical controller will see the background track if the planner is working on foreground tracks only. Finally, the "Intruder Alert" system will only work with foreground tracks. Therefore the really dangerous intruder (in other words the one you don't know anything about) will not trigger the safety system.
Great isn't it
Z