Thank you to an unnamed source. Any errors or incorrect assumptions are purely my responsibility however

A little known or unreported 25m (82ft) midget sub is visible in 2005 satellite imagery of Primorsk near St. Petersburg on Russia’s Baltic coast. There is also at least one photograph of it. The craft has a waterline length of about 23m (75ft) and has typical Russian double-hull construction. Unusually however there is no sail present in the photograph, although there are several structures on the deck.

Demystified
Since originally posting about this craft additional photos have been posted on a Russian internet site (Forums.Airbase.ru (in Russian)). The craft is confirmed to be the Carmon-18 (Кармон-18) from the CDB-Lazurit design bureau which was built in 1987 by Admiralty shipyards. It was a towed device operated until 1993 in order to test acoustic signature reduction techniques.

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The barge acted as a support facility for the Carmon-18

Any additional information or photos welcome.