It is, in fact, a supply closet. It's just that River has made herself right at home there, since the door only seems to open from the outside, which is fantastic.

She's made herself a perch out of a few crates and sits atop it, crosslegged, looking for all the world like she's meditating atop a mountain or something equally serene.

"Welcome to my kingdom. There are no exits, as you see.

We may have to resort to cannibalism."