Date: 12-04-2018 Location: Vliegbasis Leeuwarden, EHLW Type of Aircraft:Dassault Mirage 2000D Owner: French Air Force, Armée de l'air Event: Frisian Flag 2018
It's from the bell along the road on the airbase (not visible in this video). The Bell is ringing when there is airplane activity on the runway to stop the vehicle traffic on the road along the runway. In short it is a traffic light with bell ring signal.
Ok that's nice to read, this year edition of the Frisian Flag exercise they will particiate again at Leeuwarden AB, the Netherlands. They are welcome by me :)