By "empty expandable field," I'm assuming you mean that no actual documents are visible in the category when you expand it. Normally this will only occur if you don't have Reader access to any documents in the category. I believe the "Don't show empty categories" option should hide this category, if there are truly no documents there to which you have access. That's why I believe that there really are documents there, and you do have access to them. They just don't contain values for any of the fields displayed in the view columns, and so there's nothing in the view to see!
To test whether this is what's happening, expand the (non-categorized) category, and click the view row just below the category name. If there's something there to click on, then you have yourself an "invisible" document. Press Alt+Enter to view the document properties; look at the field values ("i" tab of properties dialog) and see what the Form field is and what other fields are there.
Please do not just delete these documents to get the "junk" out of your view. They're probably valuable documents; they just shouldn't appear in this view. Use the view selection formula to exclude them from the view based on their form name or other field value.