There's no visible outside lock.
You can use Farm credits to charge the inside lock, which will then hold your door until somebody spends more Farm credits than that to mutually annihilate your credits and force their entry. If they want to sneak into your room undetectably to you after the fact, they'll have to use that amount of Farm credits again to recharge the lock.
If you have zero Farm credits, or just don't prioritize spending it on your lock, anyone could just come into your room and have their way with you any time they pleased.
There's probably a grace period on that too? The Headstomper didn't mention it, but it's not like she was working from a visible checklist of what needs to be mentioned to every new inmate.