To ferret out is to search persistently until you find something hidden. Ferret out the truth.
The image is a ferret sent down a burrow to drive rabbits into the open. To ferret out a fact takes patience rather than cleverness, which is the point of the picture.
1phraseferret outTo search persistently until you find something hidden.
“It took nine months to ferret out who had signed it.”
ferret out is an action.