At ~6:15 into that episode, we're shown part of a map while two of the adventurers recant the plan. A room is clearly marked as "Goal".
Later on, at ~12 - 12:30, we see a flashback of Shiroe going over the map and the details of the plan. Presumably, what the adventurers at ~6:15 were recanting was the plan that Shiroe explains during this flashback.
Since all of the adventurers were in attendance, it would make sense that Demikas would know the map.
That doesn't explain why Shiroe didn't know what was going on when he was kidnapped though, which might just be a plot hole to add suspense and give him a reason to doubt Demikas.