Three states, not two
Workbook XML records each sheet as visible, hidden, or veryHidden. Visible sheets are the tabs you see. Hidden sheets are the ones you can bring back with a right-click and Unhide. Very hidden sheets do not appear in that dialog. They remain in the file, they can hold values and formulas, and they are normally changed through the VBA editor’s sheet Visible property or through a workbook change that sets the state back. This checker lists every sheet and its state. It does not unhide anything.
Why very hidden sheets exist
Developers hide parameter sheets, mapping tables, and old versions so a user does not edit them. Attackers hide sheets for the same mechanical reason: the dialog will not show them. The state by itself is not evidence of either intent. It is evidence that the workbook contains a sheet the Unhide command will not reveal. If you received the file from someone who said it had three tabs, and this page lists seven, you should know about the other four before you trust a total.
The repair steps live in the guide on the difference between hidden and very hidden.
What Excel does, after you know the name
On a copy, with macros disabled, open the VBA editor only if your security process allows it, select the sheet in the Project window, and set Visible to xlSheetVisible. If you cannot use the editor, a small trusted workbook can set the state, or you can ask the author for a copy with the sheet visible. Do not delete a very hidden sheet until you know whether a formula or a pivot still points at it. Deleting it turns those formulas into #REF!.
Hidden versus very hidden in the score
The Audit score treats very hidden sheets as a larger hygiene deduction than ordinary hidden sheets. Ordinary hidden sheets are common in finished reports. Very hidden sheets are less common and easier to miss. Neither deduction means the file is unsafe. The macro inventory is the place to see whether a project is present that could have hidden the sheet and could hide it again on open.
Binary workbooks
.xlsb stores the same three states in workbook.bin. This checker reads those sheet records, so visibility works on .xlsb even though formula checks do not. If workbook.bin cannot be parsed, the integrity section says so and the sheet list may be empty. An empty list with an integrity error is not the same as a workbook with no sheets. Count the tabs you can see in Excel and compare that number with the visible count here. A gap is a hidden or very hidden sheet, or a sheet the damaged package could not name. Write the very hidden names down before you unhide anything, so you know which tabs were previously absent from the Unhide dialog. That note is the review. The checker will show the same names again if a macro hides them on the next open.