The bone exists...and does not exist...somehow...?

Hi,

(using Blender 2.8[0123] on Linux)

From the internet I downloaded a rig.
I checked it for symmetrical completeness…and found two “Schroedinger-bones”: One cannot
decide, whether these bones exist or do not exist – until they are found.

The bones are clickable in the Outliner with no error message whatsoever.
The bones are part of a bone layer. The bone layer, they are part of, are painted in a darker or in
in a lighter pink. The layers are not selectable. All other bone layers are selectable.
The bones does not appear in 3D-View.
When the armature is put into edit mode then they are still not showing up.
In the top of the 3DView there is a text saying something like :
If selecting the bones in question this text shows only the name of armature.
I checked all visibility switches I know of but cannot find one, which pull these bones back into
reality…

What is going on here? :wink:
X-Files: “The bones are still out there…”
But where?
Cheers!
mcc