Spreadsheets leak differently
The document properties are the same as any Office file: author, last modified by, company, timestamps, revision count. What is specific to spreadsheets is everything present but not visible, and that is usually where the sensitive material sits.
Hidden sheets are a display setting
A normally hidden sheet is restored by right-clicking any tab and choosing Unhide. A very hidden sheet does not appear in that list and needs the developer tools, which stops a casual reader and nobody who is looking. Neither is protection, and both are reported here.
What tends to be on them
The working. Cost bases, supplier rates, margin calculations, the assumptions behind a quoted figure, staff details, an older version of the model. These get hidden rather than deleted precisely because the visible sheets still depend on them.
What checking will not find
Rows and columns hidden individually are a different mechanism and appear in no sheet list. So is white text on a white background, and cached values from external data connections. Select all and unhide from the Format menu before sending a workbook that matters.