I've been reading about similar (perhaps even identical) light leaks in Vray light maps. It is reported that "nearby" samples get picked up through thin walls. They have a switch to check for blocked visiblity that prevents it. We don't.

So anyway I figured we'd start here and see if thin slabs will leak, not just single poly planes. Here I'm at .5 inch thickness. No leak.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)