The blueprints for the sewer remodel were drawn up by Sylvester's scholars, some of whom were in Georgine's thrall, so presumably she can have a very accurate map. But.. I suppose if Ferdinand did a final check & amended it to include the temple and then handed it directly to Sylvester, this scenario could happen. All that said, connecting the fresh water tunnel to the workshop wouldn't exactly be a doorway, and while the tunnel is empty there's no reason for any final connection at all.

Rather than accepting that every incident is trug related, I think there's a possibility the attacking knights were under Magdalena's orders and they were stopped by Raublut's faction.