Ghostly child

While re-scanning this photograph, I noticed something strange near the top-left corner. I had never noticed anything there, since I had mostly worked with the cropped version of the photograph and thus ignored that whole area.

I was spooked when I found this image lurking in that corner.

This could easily be a reflection of a child that stopped on the fully lit doorway without my noticing, but the exposure was around 15 seconds. Said child would have had to stay there still for the whole time, or the silouette would be blurred. I also don't understand why the figure is inverted, nor why its reflection is straight towards the lens instead of being projected on the wall (if it came from somewhere else). I wasn't using a filter, and camera lenses are not supposed to create reflections, are they?

Pointers are appreciated.