maybe_redirect_404()

function maybe_redirect_404() {
	/**
	 * Filters the redirect URL for 404s on the main site.
	 *
	 * The filter is only evaluated if the NOBLOGREDIRECT constant is defined.
	 *
	 * @since WP-3.0.0
	 *
	 * @param string $no_blog_redirect The redirect URL defined in NOBLOGREDIRECT.
	 */
	if ( is_main_site() && is_404() && defined( 'NOBLOGREDIRECT' ) && ( $destination = apply_filters( 'blog_redirect_404', NOBLOGREDIRECT ) ) ) {
		if ( $destination == '%siteurl%' )
			$destination = network_home_url();
		wp_redirect( $destination );
		exit();
	}
}