The figure in the mirror truly departed—without doing anything extra, just as he initially said, simply "coming over to see the situation."
Vanna watched the mirror with a vigilant face for quite some time before she finally confirmed this fact and truly let down her guard.
Then, she fell into brief self-reflection and distrust.
Distrust stemmed from the professional habits of a Judge; she found it very hard to truly trust a "person" like Captain Duncan, who carried a century of terrifying legends and also had connections with Subspace, even if the other party really appeared rational and friendly, and had indeed sheltered the Plunder City-State. The reason for self-reflection was even simpler—
She realized that her vigilance had gradually weakened during the communication with Captain Duncan.
This was an unacceptable sign of weakness, something she had never expected to happen to herself.