Kate Lowe, from Dunoon, was last seen boarding the CalMac vessel at 5:50pm on Monday, May 17.
Police have now published CCTV images of Kate, purportedly showing the teenager in the queue at the ferry terminal in the town.
A spokesperson for the force said anyone who believes they have seen Kate, or have information on her whereabouts, should call 101 quoting incident 2944 of 17/05/21.
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Kate Lowe, from Dunoon, was last seen boarding the Cal-Mac vessel at 5:50pm on Monday, May 17.