The British Ministry of Defence had kept the mysterious image secret, but a copy made decades ago has blown the case wide open.
When I was a kid in the 50's we made "UFOs" with plastic dry cleaner bags filled with natural gas. We'd tie a strip of aluminum foil and a small flashlight on it, and let them go on a hillside over the city. Then we'd look for articles in the newspaper. One woman claimed it followed her car and made a humming sound. It was the height of UFO hysteria.
No idea how many reports were human-made hoaxes.