So I made a test and found that the *.sketch files in . onyx do not contain bookmarks.
In that case, they will be stored somewhere in system partition.
I remember faintly, that someone made "research" where it is stored. I forgot who and the result.
Actually I thought, that it was somewhere centrally and that it was moved to be stored locally. Seemengly I was wrong.
You could try to search this forum. It has been discussed more than half year ago.