"However, nothing says it has to be a bathroom door. Maybe it's just a narrow closet?"
If that were
the case we could all sleep better.
It's quite ridiculous if you think about it. In 90% of the Season One sets you had the space for the bathroom, we saw the door but never the inside.
Then, by Season Two, they started showing us what's behind the door
but because of the new corridor there's no more space for it.
I like the narrow closet idea, but where do our protagonists then go when they need to go?