tl;dr version: currently available Linux "Update 4" installer is missing the bonus content, use Windows "Update 4" via wine for now.

A month or so ago I saw The Darkside Detective among recently updated games at
my account and decided to check out the bonus episodes. I have downloaded the
Linux version - it was titled "Update 3" at the time - installed and played
through it with just a minor trouble: Nanny was invisible so I had to
pixel-hunt the bedroom to find her active spot. Otherwise the bonus episodes
were fun, and "BAITS MOTEL" ended in a cliff-hanger, so when the next week-end
I saw "Update 4" available - I have immediately grabbed it to check if there
is more to the story.

To my surprise the bonus episodes disappeared from the menu (see ss-lnx2.png attached),
And the "Unlock Bonus Cases" button was also gone (see ss-lnx1.png)

Okay, I have reset the game - to check if the old saves are incompatible with
the update... No luck, no bonus cases, no button to unlock them. Well,
speed-clicking the game fortunately doesn't take more than 40 or so minutes -
so I tried it, and noticed that Nanny is visible again (hooray, I guess?..)
But, well, still no bonus and no button. O-okay, maybe reset is not enough,
let's try to physically delete the data files (they are under
"$HOME/.config/unity3d/Spooky Doorway/The Darkside Detective/" by the way) and
another speed-clicking session... Still no luck.

So I started checking the things thoroughly this time. First of all, I have
noticed that I have kept the "Update 3" installer - and it's some 30Mb larger
than the "Update 4":

-rw-r--r-- 1 jeff users 197697153 Aug 14 13:34 darkside_detective_en_update_3_22933.sh
-rw-r--r-- 1 jeff users 166609894 Aug 22 12:03 darkside_detective_en_update_4_23120.sh

Then, as the file dates confirm, the "Update 3" installer was downloaded on
August 14 - and it doesn't seem to match the game changelog (last update is
"Update 4" released on July 27). Something doesn't seem right - probably there
were some trouble and the updates were somehow re-released - maybe to fix the
invisible Nanny issue?.. Okay, I have decided to dig even deeper and tried
comparing the installer contents (fortunately you just need plain old unzip to
extract them).

As it appeared, the game assets in "Update 4" are dated back to year 2017,
and, on the other hand, "Update 3" assets are dated mostly 2018, so the
"Update 4" actually ships older content - probably just the original game
files with Unity engine updated or something. And also "Update 4" is 30 Mb
shorter on uncompressed data as well - aren't those 30 Mb the missing bonus
content?

Well, file dates in the archive are not absolutely reliable, I thought, let's
check if the bonus content is mentioned in the resources. resources.assets
file from "Update 3" mentioned "BAITS MOTEL":

$ strings resources.assets | grep -i baits
BAITS MOTEL
BAITS MOTEL
*Baits Motel*

And "Update 4" doesn't (no results from grep), so, QED - there are no bonus
content in Linux "Update 4" installer.

In the meantime, the Windows version looks complete - it's data size is (more
or less) the same as "Update 3", there is an "Unlock Bonus Content" button (see ss-win1.png),
and after clicking it - the bonus episode folders (see ss-win2.png).

Looks like the game works in Wine with no major issues - so don't bother with
Linux version until it is fixed.

I tried raising the issue with GOG Support. They were incredibly slow (took
them a week to just notice the ticket and another two weeks to ask me for save
games and OS details and other irrelevant stuff) but hopefully in a month or
two they would bother to finally check the things and maybe even try to fix
them... Let's hope they are busy porting "The Witcher 3" or "Cyberpunk 2077"
to Linux. :)