esciption of the error:

When accessing remote files with nautilus e.g. with:
sftp://192.168.1.2:/home/user/, and adding this remote location as a bookmark, the bookmark shows as expected in the left panel in bookmarks. (no bug so far...)

Unmounting the location makes the bookmark disappear from the left panel.
So getting back to the remote location by klicking the just created bookmark on the left is impossible, it's no longer displayed.

.config/gtk-3.0/bookmarks stores the location like:
sftp://user@192.168.1.2/home/user/ bookmarkname

But nautilus does not show it if its uńmounted.
Manually going to "sftp://user@192.168.1.2/home/user/" in the nautilus adressbar makes the bookmark apear again. Kind of senseles once you're already there.