Murders in Ferdok - inaccessible part of the sewers

[jack_was]

Found a guide at https://www.gamebanshee.com/drakensangthedarkeye/walkthrough/murdersinferdok-2.php which shows entire Ferdok Sewer System map including especially the area inaccessible because of barred walls and a collapse.
I failed to sniff out any fissure wall with Forgrimm's Dwarfnose at maximum level (the area ) and tried to outsmart the cultists by taking the left corridor before they cause an explosion in the tunnel they took as their escape route. Unfortunately no matter how I try to approach this in-game event (running at full speed or sneaking) they always manage to block the passage with debris and when you see them again in the parallel path the door they are assumed to get through are already locked dead.
I wonder how did the authors of the aforementioned guide manage to map an inaccessible part of the sewers because they haven't mentioned anything about it and it's obvious you can't reach these places by boosting Perception.
Has anyone found a trick to visit this mysterious area?

[Oneeyed]

There are 3 secret doors in the sewers. And you dont miss out on anything behind the closed doors. There is either absolutely nothing behind it or you can get there another route. You can see the routes on the map.

Also you have all the time in the world. You can explore the other part of the sewer while you are following the assassins.

[jack_was]

Originally posted by Oneeyed:

There are 3 secret doors in the sewers.

Yes, but they don't lead to the area I'm talking about.

And you dont miss out on anything behind the closed doors. There is either absolutely nothing behind it or you can get there another route. You can see the routes on the map.

I can't find any way to get there - thought about connections with both areas behind fissure walls but failed. Also tried to attack the cultists before they cause an explosion but I'm unable to interact with them in this way - only talking and pickpocketing are available alas you can't get close enough to accomplish any of these tasks.
How can you know that there is nothing to collect on the route the assassins took?

[NavFamG]

Depending on where you're at in the story all the sewers might not be open to you.

In this case the quest wants you to be blocked by that explosion and not find a way around it.

Later on you'll find a locked door in a tower and will stay locked until you've passed the required milestone.

Things like this are just a devs way of funneling you towards the path they have the story set for.

[jack_was]

From what I know you can't visit this part of the sewers again since the entrance from Wagoner's Home is locked dead. In theory the corridors end at archivist's residence but there is no chance to get inside till the inquisition guards the entrance.

[Oneeyed]

As I wrote there is nothing behind the doors you cant open. There are some doors you can get behind if you just follow the way then you get behind it. There purpose is just to make your way longer and for getting behind one door you have to find the secret door

https://drakensang.fandom.com/wiki/Kanalisation?file=Kanalisation.png

This map is in german but the green lines are the doors you cannot open. The red lines are secret doors. As you can see you get behind 3 of the locked doors.

And also you cannot enter this part of the sewers again after you were there. There was a way to reopen it by mod but I dont remember the sql commands for doing it