The bookmark decoration square bracket is visualized in a previous table cell if the parent table does not have content. 
The visualization of the ending bracket does not affect the location in the document of the bookmark range end.

Steps to reproduce:
Insert an empty table 3x3.
Select the entire table.
Insert bookmark.
Toggle the bookmarks. RadRichTextBox has a ToggleBookmarks method which will visualize or hide the bookmark surrounding square brackets.

You can also check the attached screenshot.