A security guard hears footsteps in an empty corridor every night but never sees anyone. What could cause this phenomenon?

Category: Mystery · Difficulty: hard

Hint: Consider sounds traveling from far away locations.

Answer

Echoes from a distant construction site

Explanation

Construction noises or passing trains create echoes that sound like footsteps in the empty hall.

This is a hard mystery riddle — a tricky one that takes a little thought. The answer is Echoes from a distant construction site.

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