After setting GOBJECT_DEBUG=objects comparing the outputs
of the following program run with ITERATIONS set to 2 and 1:

#include <gtk/gtk.h>

#ifndef ITERATIONS
#define ITERATIONS 2
#endif
      
int
main (int argc,
      char *argv[])
{   
  int i;
  if (gtk_init_check (&argc, &argv))
    for (i = 0; i < ITERATIONS; ++i)
      { 
        GtkWidget *window   = gtk_window_new (GTK_WINDOW_TOPLEVEL);
        GtkWidget *notebook = gtk_notebook_new (); 
        GtkWidget *page     = gtk_drawing_area_new ();
        
        gtk_container_add (GTK_CONTAINER (window), notebook); 

        gtk_notebook_append_page (GTK_NOTEBOOK (notebook), page, NULL);
        gtk_notebook_set_scrollable (GTK_NOTEBOOK (notebook), TRUE);

        gtk_widget_destroy (window);
      }

  return 0;
}


the number of objects alive at exit increases for the presence
of few GtkBuiltinIcon objects.

The problem is here:

https://git.gnome.org/browse/gtk+/tree/gtk/gtknotebook.c?h=gtk-3-22#n1843

in gtk_notebook_finalize priv->{,header_,tabs_,stack_}gadget are cleared,
but priv->arrow_gadget[0-4] not.