mystery case; strange incident
Refers to a puzzling or bizarre event, often with criminal or supernatural undertones. Common in news and fiction.
The police are continuing the investigation of the mystery case.
Strange incidents have been occurring one after another in this town lately.
Compound of 怪 (kai, 'mysterious') + 事件 (jiken, 'incident, case'). The kanji 怪 carries a sense of suspicion or strangeness.