Film from camera back

Whatever the smokey artifact was that showed up in three of my digitile camera shots did not come through on the camera using regular 35mm film of 200 speed. I still do not know what caused the anomaly. The film shots were abnormally dark. My wife has made a joke of the many pictures that I have shot of those stairs since one shot showed what might be a "ghost-like" head of a woman (A lot of debate on whether its a face or not). Because of these many shots I have a really good expectation of what a picture of those stairs should look like. These pictures are much darker than they should have been; however, I realize there could be several possible explainations. I would think that my next most logical date for trying to photograph something unusual will be Sept. 21st. tomorrow., the autumn equinox. But the next best date is Halloween which is fast coming.