Take a look at the last shot of the 4-side (d-side) at the roof level. The owners of this building sealed up the shaft at the roof what appears to be nothing more than a sheet of plywood. The weight of a fully geared firefighter would almost certainly fall through this if they stepped on this “feature of the building”.
Take a few seconds to give the building a look from the ground before heading to the roof. This dangerous feature would be invisible if looking at it on the roof. It will look like a substantial roof from front to rear, and you will have no way of knowing about the presence of the shaft on the 4-side.
One other tell-tale sign (not present in this photo) deals with the parapet wall. If a parapet wall runs front to rear, and there is a 10-15 foot opening in that parapet wall, this may be a sealed up shaft area.
Stay away from these areas while traversing the roof! They are deadly.