code: private String _getVisitId(UIComponent component)
    {
      // We first check to see whether the component's id
      // is in our id collection.  We do this before checking
      // for the full client id because getting the full client id
      // is more expensive than just getting the local id.
      String id = component.getId();
  
      if ((id != null) && !_ids.contains(id))
      {
          return null;
      }
  
        // The id was a match - now check the client id.
      // note that client id should never be null (should be
      // generated even if id is null, so asserting this.)
      String clientId = component.getClientId(getFacesContext());
      assert(clientId != null);
  
      return _clientIds.contains(clientId) ? clientId : null;
    }
docs: If so, returns its client id.  If not, returns null.