Footsteps on the Roof

Old Li lived on the seventh floor, the top floor. For the past month, every night at 2 AM, he could hear heavy footsteps, 'thump, thump, thump,' coming from the roof, as if someone was running up there late at night. Old Li was driven to near madness and reported it to the property management several times. They told him the rooftop door was always locked and no one could possibly get up there. This night at 2 AM, the footsteps sounded right on schedule again. Enraged, Old Li grabbed a flashlight and an iron pipe and rushed straight to the rooftop. The heavy padlock on the iron gate leading to the roof was already rusted and caked, clearly having not been opened in years. Old Li struggled to break the lock and pushed the door open. The rooftop was eerily silent, empty. Old Li shone his flashlight on the ground and suddenly noticed thick dust covering the concrete floor. And on the dust, there were only the footprints he had just made. He breathed a sigh of relief, thinking he was hallucinating, and prepared to go downstairs. Just then, he habitually raised his flashlight to shine on the ceiling – A clear set of footprints was hanging upside down from the ceiling directly above him, extending from the rooftop door all the way to the spot he was currently standing. The starting point of the footprints was right on his shadow.