- Attach any file as worksession
- Detach it again. The file will still be visible in the layer manager.
Attaching/updating it again will leave more and more orphaned layers.
Orphaned layers also remain not only with worksessions but when linking external blocks (xrefs), and removing them again (using block manager > delete).
Maybe all this becomes obsolete/fixed with the Layer panel rewrite… It kind of a nuisance in R7, anyway.