Hidden cells in Excel
Hidden cells in excel are often achieved by colouring the contents of the cell (font colour) the same as the fill of the cell. In effect you can’t see the contents even though it is there and will be used if referred to.
This technique has risks as users may accidentally overwrite the contents without noticing. To spot these ‘hidden’ cells you should make use of a spreadsheet testing tool like Spreadsheet Professional. Another way is to highlight a whole section and you will notice the ‘hidden cells’ in the shaded areas (typically the white will show up in the shaded colour).
If, however, you are using Excel 2013, this feature seems to have been removed. So be careful in Excel 2013. Just because it looks blank doesn’t mean it is blank.