@Override
  public void unregisterTestHook(MembershipTestHook mth) {
    getDistribution().doWithViewLocked(() -> {
      if (membershipTestHooks != null) {
        if (membershipTestHooks.size() == 1) {
          membershipTestHooks = null;
        } else {
          List<MembershipTestHook> l = new ArrayList<>(membershipTestHooks);
          l.remove(mth);
          membershipTestHooks = l;
        }
      }
      return null;
    });
  }
This is an event reflecting that a InternalDistributedMember may be missing but has not yet
left the system.
geode-core/src/main/java/org/apache/geode/distributed/internal/ClusterDistributionManager.java