Sheet absent from the unhide dialog

How the text is hidden

The same <sheet state> attribute in xl/workbook.xml takes a third value, veryHidden. A sheet in that state is not listed in Excel's Unhide dialog, so no sequence of clicks in the application reveals it — restoring it requires the VBA project window or editing the XML. The worksheet part and its cell values are otherwise ordinary.

Why a model still reads it

Nothing about veryHidden touches the worksheet part. Every library that walks the package — and every text extractor built on one — reads the cells without consulting the workbook's sheet list.

What we do about it

cellReason maps state="veryHidden" to container-undiscoverable, which is graded strictly above an ordinary hidden sheet. The rule xlsx-very-hidden-sheet fires at severity high with action strip: the text is removed from the canonical text handed back to the caller, on the grounds that no human ever saw it. visibilityOf returns never for this reason — the switch has no default, so a new concealment mechanism cannot silently fall through to "the reader saw it".

How often it fires

0% of 200 real workbooks from five national open-data portals (CA, IE, DE, GR, UK), measured 2026-08-21.

This is an alert-volume number and nothing else. It says how often the alarm sounds on documents as found — not how often it is right, and not whether what it found was harmless. Documents as found may themselves carry concealment. Read it against the population named above rather than as a property of documents in general.