They may be what are known as shadow people. The following exert is not my own work and all of it was obtained from a site containing information on shadow people. I have copied and pasted the entire article.
Shadow people are paranormal shadow-like creatures that people usually see in their peripheral vision. Reports suggest that they do not appear to reflect light and that their appearance is essentially a silhouette in black.They usually have no discernible features beyond the general outline of their form. However, shadow people have been reported to have shifted into a more human form and in these instances it is said that sometimes features such as glowing eyes and sometimes mouths or nostrils are discernible.Reports of shadow people, sometimes called shadow folk, shadow men, or shadow beings, have many similarities with ghost sightings, so many similarities that many paranormal experts consider ghost and shadow people one and the same.However, other experts disagree and claim that shadow people are a separate phenomenon from ghosts for a variety of reasons.First, they claim that the movement of shadow people is said to be quick and jerky, sometimes with stops, starts, and changes of direction, not at all like the smooth floating motion often associated with ghost sighting. This is apparently the primary reason they are usually seen out of the corner of the eye.Second, some people report being able to discern that the shadow people are wearing a fedora style hat like a 1930?s-era gangster or a cloak.Third, shadow people usually appear quite solid, unlike the ephemeral appearance of ghosts.Fourth, there are very few reports of positive interactions with shadow people.
Unlike the friendly ghost sightings that are fairly common, encounters with shadow folk are almost always frightening or shocking. In fact, even the experts that view shadow men as a subset of ghosts, usually concede that shadow men are a malignant beings.The supporters of shadow people as a separate and unique phenomenon offer all sorts of speculation about the nature of shadow people using ideas from religion, parapsychology, metaphysics, and the occult.One of the many ideas is that shadow people represent a Thought-form, ghost or demon that was created by extraordinary pain, suffering, and trauma in a dying persons life. Others suggest that shadow folk have been purposefully summoned from another realm through black magic or other occult practices.The fact that they do not seem to reflect light has led some to theorize that they are from an alternate universe with different laws of physics or perhaps they are caught in a trap between two physical universes. David Icke has proposed that shadow people are related to Grey aliens, or to the Reptilian humanoids.The theories about the nature of shadow people are almost endless, but they are certainly quarry that should only be sought out by experienced, well equipped ghost hunting teams ? never by solo hunters. [/quote]
The information was found at http://www.ghost-hunting-equipment.com/shadow-people/