Have you ever see someone out of the corner of your eye, but when you look full on, there is no one there? I have done that several times, including on investigations. It happens a lot at Gretchen's Lock, for example. Definitely haunted there, as I am sure most who have been there will agree.
The question is, did you see something that may be paranormal in nature, or was there something in your sight that might resemble a human form? One thing I do when it happens to me is check out the area and see what is around me. Is there anything there that I might be mistaking for a person? It could be a telephone pole or a tree, or maybe a coat rack or lamp. Keep in mind that I also make an attempt at writing down what I saw with as much detail as possible, as soon as I can. Even if there is something that I might have mistaken for someone in that sideways line of sight, I still write it down. The next thing I do is research. I ask questions of my other crew members to see if they saw anything out of the ordinary (not exactly what I saw). I also check the history of the area to see if there is any other recollection of anything like what I saw.
If you detailed seeing something that is commonly seen in that area, like the Bride of Sprucevale, Esther Hale, would be a woman in white, possibly in a wedding dress, haunting the bridge or Grist Mill, then perhaps you did see something. Or if you keep seeing a little old lady wearing a yellow dress and shawl, sitting in a rocker and you later discover a little old lady died in your home years before, you could be seeing something. The key word in all of this is RESEARCH. You won't know unless you look. Never assume anything when you are dealing with what might be ghosts. We don't know enough about what they can or can not do to say with any certainty that what you saw in the corner of your eye was the coat rack. My theory suggests that a ghost uses whatever it can at times to get your attention. More often then not, if you do do the research, you will find evidence to back up what you saw. Try it the next time you see something out of the corner of your eye. What can it hurt?