General comments:

Ok you had two choices. Either take the ladder back up to We Can See In The Dark... Can You? or take the other ladder.

So you took the other ladder and now you're in the Labyrinth. But something in the back of your mind should be troubling you.

Remember what the first Dead German on Happy Happy, Carnage Carnage said:

They never came back. Muller never brought
any of them back. They're all dead, even
Muller.

and...

Everybody who left after I died had an
MP-41. If you want one, just look for their
bodies. They never came back, so they're
rotting around here somewhere ...

You found four bodies but only three MP-41s. What happened to the fourth MP-41? Did someone take it?

Then Friedrich revealed that Walter and someone else had gone in search of a strange wooden box:

They killed us all. The last four members of
the expedition, unless Walter and whoever
went with him survived.

and...

Muller gave Walter a gold key and sent him
to recover a large amount of gold and a
strange wooden box.

Did Walter or the other German take the missing MP-41?

These last two Germans brings the total count to eighteen which is the figure Behrens gives us on Wrong Way!:

There were eighteen of us, I think, and I
doubt anyone left alive. I never saw anyone
come back this way.

However Behrens never saw anyone come back up. Since he was lying near the base of the ladder leading up to Ground Floor he was well placed to know if anyone did.

So did Muller send Walter and another man to recover the wooden box BEFORE the others went down to Wrong Way!?

If so who took that missing MP-41 on Happy Happy, Carnage Carnage?

Everybody who left after I died had an
MP-41. If you want one, just look for their
bodies. They never came back, so they're
rotting around here somewhere ...

Nobody came back?... Nobody came back?... Nobody came back?

Decision to make:

Carry on regardless.
Head back down to Happy Happy, Carnage Carnage.