Show a unique message on content restricted for a hidden membership level.

hidden_level_content_pmpro_text_filter.php

<?php
/**
 * Show a unique message on content restricted for a hidden membership level.
 *
 */
function hidden_level_content_pmpro_text_filter( $text ) {
	global $post;
	
	$access = pmpro_has_membership_access( $post->ID, NULL, true );
	$level_ids = $access[1];
		
	if ( is_array( $level_ids ) && in_array( 9, $level_ids ) ) {
		$text = '<p>The respective Membership Level was currently inactive.</p>';		
	}

	return $text;
}
add_filter( 'pmpro_non_member_text_filter', 'hidden_level_content_pmpro_text_filter', 5);
add_filter( 'pmpro_not_logged_in_text_filter', 'hidden_level_content_pmpro_text_filter', 5);