Just to demonstrate the lighting issue, here's a photo I took this afternoon.  You can see inside the building but you'll notice it's very dark, and the graininess shows the trouble the sensor had picking any light.  But notice that in order to get some pickup from the inside the sensitivity is too high for outside, so the concrete in front of the door is blown out.

Unlike BB's tunnel example, there are lights inside: you can see the overheads and trust me, there's lots of light to see inside (I was working in the hall with sunglasses on).  If the lights had been out, you wouldn't have seen anything inside at all very far beyond the door.