I'm having, what I feel, is a similar issue.
I have, what LOOKS like a comment box containing text on my excel.
It's only view-able when I select certain cells (All the cells selected have the same comment--not sure if it's coincidence or copy/pasta.)
I can move around the "comment" but cannot delete it. Pressing "Delete" on my keyboard clears the cell selected.
Going into design mode yields no results.
Here is an example of the document. I can email it upon request.
Thank you in advanced!