指透露不应该让别人知道的秘密to let the cat out the bag

• 张作霖后来向陈文运泄露天机。

  Zhang Zuolin later divulged the secret to Chen Wenyun.

• 正是这张“伪造”的童年合影泄露天机。

  It was this "fake" childhood photo that let the cat out the bag.