Sources revealed that the cockpit crew, the captain, wanted to show the specific squawk codes for emergency situations to some junior intern. So they entered 7500 and this was the response. Bad idea.
The F16 remark I made earlier was meant to indicate that the police/defense system in place did not take any half measures. All units were put on alert. By the way I live near the air base (southern NL) that scrambles these fighters and I can confirm that all fighters have returned to base at 2000Z.