Have you ever been driving down a country road at night when there is little traffic and seen a strange object ahead.  Maybe you see a giant spider, a wolf, a witch, etc.  Then when you get closer you see that it is just a pile of rocks, a tree stump or a bush?  Almost everyone has had something like that happen at one time or another.  Has it ever happened when there is a lot of traffic?  Probably not.  It just doesn't happen when there are other people nearby.  Why?  Some would say it is simple because your getting a better view of the thing with other cars on the road, but that is not always true.  I have a theory of my own.  
    I think what happens is that when you see something your brain can’t resolve, it subconsciously send out a request to others asking "what is that?".  If other people are near the spot, or just passed it, you will get a reply.  If there is no one nearby to send a reply, your brain is on it’s own.  That is when imagination takes over and you get the strange image.