My thinking was that Darvin was a relatively new assistant to Barris, and pretty much was snuck in via an interview.^^^
IDK - The Klingons went through (and put Voq/Tyler) through great pains to hide the fact he'd been transformed.
All it too for Arne Darvin to be discovered was McCoy doing a standard scan with his Medical Tricorder.
Barris, being the overconfident buffoon, saw the paperwork in order and didn't bother with anything else.