Well, I'm not sure what's causing the light inside your setup but i doubt it's indirect light. Even if it is indirect light, it should not have been there in an enclosed and completely unlit environment.

There's a theory i can come up with though..
We know from TA experiments that there's a problem with the brycean cube's surface normals that make one of the cube's faces react strangely to light, maybe this affects the HDRI example scene too?

But yea, I think it should be a bug. Light, direct or indirect, should not be inside an enclosed non-transparent environment with no light source inside it.

(_/)
(='.'=)
(")(")This is Bunny. Copy and paste bunny into your signature to help him gain world domination.