gistfile1.php

<?php

function invman_generate_location_tag( $term_id, $term_tax_id ) {
	$existing_tag = get_term_meta( $term_id, 'invman_location_tag' );
	if ( $existing_tag ) {
		return;
	}

	$characters        = '23456789ABCDEFGHJKLMNPQRSTUVWXYZ';
	$characters_length = strlen( $characters );
	$random_string     = 'B';
	for ( $i = 0; $i < 3; $i++ ) {
		$random_string .= $characters [ wp_rand( 0, $characters_length - 1 ) ];
	}

	/**
	 * Now check if this tag already exists
	 */
	$existing_tags = get_terms(
		array(
			'taxonomy'   => 'inventory_location',
			'meta_key'   => 'invman_location_tag',
			'meta_value' => $random_string,
		)
	);

	if ( $existing_tags ) {
		return invman_generate_location_tag( $term_id, $term_tax_id );
	} else {
		add_term_meta( $term_id, 'invman_location_tag', $random_string );
		return $random_string;
	}
}
add_action( 'edited_inventory_location', 'invman_generate_location_tag', 10, 2 );
add_action( 'created_inventory_location', 'invman_generate_location_tag', 10, 2 );