Wiktionary
vb. 1 (&lit: English) to come out of hiding; to become visible. 2 (idiomatic, by extension) to disclose one's real thoughts and intentions.
Usage examples of "break cover".
And he could never break cover to make contact with a civilian while he was on a mission as important as this one.
A moment later he saw her break cover and run toward him, carrying her recording equipment.
But then he had seen him run down Thierry, a Thierry screaming for help, about to break cover under the eyes of the Germans.
Mother Joanna sulked then until she saw a stag break cover at the edge of the forest.
My men, whom I had instructed to keep close together and sing from the time they left the camp until they joined me on the forest road, were not due for an hour and a half, and during this time it was more than likely that the tigress would break cover and try to stalk or rush me.
The Home Office is confident that, sooner or later, the gang will break cover as they forcibly recruit new members, or as the leader's megalomania carries him over the brink into delusions of omnipotence.