An ace in the hole is an argument, plan or thing kept hidden until needed.

VOA: special.2009.11.22

That's the thing; it was hidden by the board.

这个表达式被黑板挡住了

耶鲁公开课 - 博弈论课程节选

And I knew at the same time that in the end, at the last minute, Nunu would in fact lock away the silver, and that later still, once the silver was forgotten, when we discussed the whole thing, the thing that could not be hidden, Nunu would be somewhere nearby with her keys, in her best black dress, with her wrinkles, with her silent, observant caution.

NPR: Excerpt: 'Esther's Inheritance'

Most people start at the bottom, playing gigs in pubs and clubs, writing songs and sending demos to record labels in the hope that someone, somewhere will spot the hidden talent and sign them up as the next big thing.

CNN: The Quest for music

"This only points to one thing: The Marcos hidden wealth is not hidden anymore, " he said.

CNN: PEOPLE