The exit of the underground river from the canals of Oksenfurt to Pontar

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When we go through the quest "The Great Escape" in the third chapter of the main story, according to which Geralt rescues Marguerite Lo-Antil from the Deira prison in Oxenfurt, in one of the prison cells you can find the "Discarded Note".
In it, an adventurer lost in the canals tells that an underground river flows into the Pontar somewhere. In this underground river, he throws a note in the hope that the note will be read by someone.
I did not manage to find this outlet of the underground river in Pontar.
Question: is there really an exit from the canals to the river in Oxenfurt? Or is it just the intention of one of the authors, which was not implemented?
 
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