Someone there when there is no one there

I took this picture on my way home this Halloween weekend and tried to post it to facebook.  It was late at night and this area, which was normally crowded with people in the park during the day, was completely empty and serene.  When I tried to post the photo, facebook registered someone present and sent a notification asking if I wanted to “tag” the person in the photo as someone I knew.  Try as I might, I could see no one in the picture.  When I took it, I thought I was completely alone, but now I am not so sure.