I reported FogBugz report #1143875 on this, but I figured I'd post it here. The bug report contains a repro project, but the basic idea is that Occlusion Culling seems to be flaky with PB objects. I can start with a single static PB object and everything's fine. But when I start adding additional objects to the scene, the Occlusion Culling bake starts seeing through the PB objects. Here's an example from the repro project showing how the camera can see out of the room on the left, despite it being static. Some of the visibility lines are blocked, but many of them make it out. I'm not 100% this is necessarily PB related, but I've only been able to reproduce this behavior on PB objects. Other Unity primitive meshes, or other FBX meshes, seem to work fine. I'm not sure whether this behavior is a weird interaction between PB and the occlusion culling system, or whether this indicates something else is going on.