If you let the cat out of the bag, you reveal facts previously concealed.
From Labour Press :
“David Cameron’s close friend and Cabinet colleague Oliver Letwin has let the cat out of the bag.
“He’s been caught letting slip what’s on the real Tory agenda for a second term – the old right-wing idea of a flat tax championed by George Osborne ten years ago.
“Not content with having already cut taxes for millionaires, a Tory flat tax would mean even lower taxes for the richest and higher taxes for millions of working people.” July 2014
We tried to organise a surprise party for Tom, but unfortunately his wife let the cat out of the bag and spoilt the surprise.
Synomym : to spill the beans