He moves the black nut outside of the door.
“He would, without a doubt, try to leave the safe. Not only because it only opens from the outside, but also because Lancelot is the only one who knows the password. If he locked himself in there, no one would be able to save him. To top it off, the safe would be out of oxygen in 5 minutes, so he’d guaranteed to kick the bucket before he could even consider calling a locksmith. Therefore, he’d be forced to leave the vault, wanting to or not. That’s how we make him let go of the Grail.”

Chaldea Case Files: Manhattan, Dark Crime Skyscraper