I'm sure the guy isn't on duty 24/7.
[citation needed]
That is not what happens in every video game I have seen.
Have you sat there for over 24 hours, watching the whole time to make sure there isn't a shift change?
I am totally doing this if I ever make a video game of this type.
No-kills walkthrough:
Go straight, then turn left, open the door, hide in the box close to the guard looking straight at the next door with his back to you, leave the game running 24 hours sharp, open the door after the guard leaves, go straight again...