@Override
  public void fromData(DataInput in,
      DeserializationContext context) throws IOException, ClassNotFoundException {
    coordinator = context.getDeserializer().readObject(in);
    senderId = context.getDeserializer().readObject(in);
    coordinatorPublicKey = StaticSerialization.readByteArray(in);
    rejectionMessage = StaticSerialization.readString(in);
    isShortForm = in.readBoolean();
    if (!isShortForm) {
      fromView = in.readBoolean();
      networkPartitionDetectionEnabled = in.readBoolean();
      usePreferredCoordinators = in.readBoolean();
      view = context.getDeserializer().readObject(in);
      registrants = GMSUtil.readHashSetOfMemberIDs(in, context);
    }
  }
When the response comes from a locator via TcpClient this will return the locators member ID.
If the locator hasn't yet joined this may be null.
geode-membership/src/main/java/org/apache/geode/distributed/internal/membership/gms/locator/FindCoordinatorResponse.java