Display custom field outside the loop

by 

<?php
  define('WP_USE_THEMES', false);
  require('wp-blog-header.php');
?>

<?php
    $post_id = 288;
    echo get_post_meta($post_id, 'caption', true);
?>

That part is enough – are you sure that this is the name of your custom field and the path to wp-blog-header is resolved and valid?