I tried [13.0.2] 2021-02-25 , and the book toggle , if the file is not included in the explorer , does not work.
Just open an outside file , and try to book mark any line, It does not work.
but if the code file is included, in the project/explorer , the book works without an any issue
Seems to work for me.

Hi @gbil-sys ,
Are you using Windows, opening the _external file_ from a different drive than the folder you opened in VS Code?
I mean, you open a folder in D: but the file is from C:, or vice-versa?
As @fade2gray reported, it is working, for me too, but I could reproduce an error while doing that _different location_ scenario. On MacOS I had no issue using files from a different device.
Hope this helps
ps. @fade2gray, thanks for testing 👍
Hello @alefragnani
Thanks a lot for your response. I have done further troubleshooting on this issue. I am on a windows system and using subst command to map a local folder to a drive letter N:. The vscode is confused between C:\home\working directory and N:. Any time I am opening a file from N: drive, it does not show a bookmark but adds a big ///////////////////////////////////n:// in the "BOOKMARKS: explore" and bookmark does not work.
I tried to reproduce this after removing the drive mapping but this issue does not show anymore.
* Another feature request. Could you please make "BOOKMARK: Explorer" window full link option configurable?

Hi @jbengineer ,
If you use subst, even if you open _the same physical folder_, VS Code will understand as two different workspaces, so it will save/load different set of bookmarks.
If you open the folder from N:, but then for some reason open a file using the C: notation, it will add a bunch of / because that file will be _relative to the current location_ (../../../../c:/../.. and so on). But, if instead you are open the folder from N: and open a file from the same N:` location, it _shouldn’t_ do this.
About your request, the link doesn’t display _full path_ if the file is _inside_ the workspace/folder you opened. I’m not in from a my computer right now, but I guess it’s the same behavior VS Code has. I see the _full path_ is important for files _outside_ the opened folder/workspace, but I understand it can be hard to read. Feel free to open a new issue about this request.
Hope this helps
Are you using Windows, opening the _external file_ from a different drive than the folder you opened in VS Code?
I mean, you open a folder in D: but the file is from C:, or vice-versa?
I have the same issue
Despite my earlier comment, I have just discovered that I can't create bookmarks in a file stored anywhere other than on the C:\ drive - not on a different partition on the same physical drive as C:\ (D:) or on a removable drive etc.