I had a similar experience; not sure what the culprit was but I was trying to walk to my car at night while visiting grandparents deep in the woods. It was fairly dark and I only heard flapping but I could tell it was BIG by the sound and freaked me the fuck out

It got to my lizard brain and after trying to work up the courage to leave the safety of the front deck for a good ten minutes, I decided to head back in and just stay the night