Actually, it'd be quite easy to tell the difference between a physical knock and a recorded knock. In an old mansion-style house with heavy wooden doors, a real knock will resonate quite deeply through the wood, causing a perceptible thump that a tape recording will be able to replicate only in sound (since the tape won't cause the door to vibrate). So the idea that no one would mistake a real knock for a recorded one is fairly sound. Even a layman would be able to sense immediately that they were not witnessing an actual knock.
The question then I guess would be why lie about it.
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