ThreeJS | Detect when object leaves another object

I'm making a ThreeJS project in which I have planes (Object3D) flying inside a sphere (Mesh).

I'm trying to detect the collision between a plane and the border of the sphere so I can delete the plane and make it reappear at another place inside the sphere.

My question is how do I detect when an object leaves another object ?

The code I have now :

detectCollision(plane, sphere) {

  var boxPlane = new THREE.Box3().setFromObject(plane);
  boxPlane.applyMatrix4(plane.matrixWorld);

  var boxSphere = new THREE.Box3().setFromObject(sphere);
  boxSphere.applyMatrix4(sphere.matrixWorld);

  return boxPlane.intersectsBox(boxSphere);

}

In my render function :

var collision = this.detectCollision(plane, this.radar)
  if (collision == true) {
    console.log("the plane is inside the sphere")
  }
  else {
    console.log("the plane is outside the sphere")
  }
})

The problem is that when the planes are inside the sphere I get true and false basically all the time until all the planes leave the sphere. At that point I have a false and no more true.