下落不明

be of unknown whereabouts

含意不明。

The implication is unclear.

来意不明

have no clear reason for coming

国籍不明

be of uncertain nationality

不明国籍的飞机

aircraft of unidentified nationality

不明事理

lack common sense

不明是非

not know right from wrong

不明底蕴

not know the finer details

不明真相的人

someone who is ignorant of the truth