unexplained
adjective uk/ʌn.ɪkˈspleɪnd/ us/ʌn.ɪkˈspleɪnd/
（事件、行为等）未说明的，未解释的，原因不详的
Unexplained events, behaviour, etc. are ones for which people do not know or understand the reason.
an unexplained death/noise 原因不明的死亡／噪声
He was shot dead earlier this year in unexplained circumstances. 他在今年早些时候莫名其妙地被人开枪打死了。