title: Correct HTTP status code for existent resource, but non-existent entity?
body: <p>Say the client is requesting the following URL:</p>
  
  <pre><code>/user-details?user=123
  </code></pre>
  
  <p>If <code>/user-details</code> was a non-existing resource, the correct status code would obviously be <code>404</code>.</p>
  
  <p>However if <code>/user-details</code> does exist, but no user with id <code>123</code> exists:</p>
  
  <ul>
  <li>I've so far returned a <code>404 Not Found</code>, but experience has told me that it makes it confusing to not know whether it is the <em>resource</em>, or the <em>entity</em> that was not found;</li>
  <li>I've considered using <code>400 Bad Request</code>, but I find it confusing as well, as the request is technically correct, just requesting a non-existing entity.</li>
  </ul>
  
  <p><strong>Is there a more suitable HTTP status code for this purpose?</strong></p>