Because a synch issue (I think, and maybe my fault!) I have lot of notes duplicated, and the duplicates has no notebook link.
Feature suggestion:
1.- In configuration, add a switch (on/off) to locate automatically all these notes and assign them a new notebook like "no-notebook" so I can check them easily.
2.- In search, as "-tag:*" can be used to search notes that has no tags, add "-no-notebook" or "-notebook:*" to do the same for notes that are no linked to any notebook.