We have been riding HM for 40 years and I have NEVER seen or heard of this. How COOL that you experienced this!
I would think that at some point, especially during Halloween, we would have caught this effect. Back in the 80's, before Mickey's Not So Scary Halloween Party, MK would put on a Halloween "show" at HM. They would have an actor dressed as a scarecrow standing perfectly still. People would gather around him and wait to catch movement. They also had a werewolf walking through the cemetery and around the outside of the building with a plastic rat hanging in his mouth. I was about 13 and I remember the werewolf following groups of people into the ride and scaring the you know what out of them! He saw how scared I was and waited for me and my family at the exit behind one of the stone pillars as you come out. I didn't see him and he got behind me and put his hand on my shoulder. My dad said the blood drained from my face when I realized it was the werewolf. I ran all the way to It's a Small World!