[CJFusco]

Hmmm that is bizarre. What made me initially think "shadow sighting" was that you saw it out of the corner of your eye - a common scientific explaination for "shadow people" is Pareidolia (from the Wiki):

"In most instances, witnesses report seeing shadow people in the peripheral areas of their vision. This areas of vision is linked to the areas of the brain that recognizes patterns, however it provides less detail to the brain than Macula (Center-forward) vision. This can lead to a condition know as pareidolia, in which the brain incorrectly interprets random patterns of light/shadow or texture as being familiar patterns such as faces and human forms.[2] The same condition can also be observed in Macula vision in low light conditions, or when viewing a complex but random image. A common example would be perceiving a shadow, thrown by an item of furniture in a darkened room, as being a person."