Hiren Lake Legend Murder Case: File 3 - S1-E6
Plot hole: Kindaichi's denouement can happen only because the killer misses half a dozen people leaving the cottage covertly; Kindaichi does not have a clue of the killer's observation point, and despite what the detective says, the killer was not just listening, but watching them too with binoculars. Moreover, moving a microphone sensitive enough to capture the sounds of the whole room would produce noises which would have alerted the killer that they were tampering with it.