In the UK (Swanwick AC/TC and Prestwick) can see Selected Flight Level on their radar screens. This was a cartainly a major contributor to the reduction of Level Busts within the London TMA. You input it wrong, they will question you.
Assuming the French ACC has this, it would have been obvious rapidly that the SFL was zero! Without knowing the intentions of the flight (no RT response), the Air Force would no doubt have been informed rather quickly!