I see something in Digitaal Hoogtemodel Vlaanderen II, Skyview factor 0,25 m but nothing on Strava headmap , i already spoted the perimeters in the past and from Tenreuken or Charel-Albert there is no entry ; i guess that if there is a entrance it's near https://www.openstreetmap.org/node/255683190 but i think it's restricted. I can make a detour next time i pass by there.