Final answer:
The miniature was stolen and placed in Justine's pocket; the thief expressed guilt over the action.
Step-by-step explanation:
The miniature got into Justine's pocket because it was taken from someone else's pocket and placed there. The text suggests that a character is admitting to the act of theft and experiencing great distress over it. The miniature was stolen as the person leaned against bricks and the culprit transferred it into Justine's possession without her knowing.