de aap komt uit de mouw

Etymology
Literally, “the monkey comes out of the sleeve”; it likely originated from the use of trained monkeys by performers.

Proverb

 * 1) The truth is revealed; a former secret has become known; often in relation to something shameful or furtive activity.

Usage notes
Often used with nu or daar in the first position: nu komt de aap uit de mouw; daar komt de aap uit de mouw.