Spreadsheets leak differently
The document properties are the same as any Office file: author, last modified by, company, timestamps. What is specific to spreadsheets is everything that is present but not visible — hidden sheets, hidden rows and columns, and cached values from external connections.
Hidden sheets are reported, not removed
A hidden sheet frequently holds the working that produced the numbers on the visible ones: cost bases, margins, the model behind a quoted price. This page flags them clearly, but does not delete them, because removing a sheet destroys data and that decision has to be yours.
Before sending a model to a client
The safest habit is to produce a fresh workbook with only what the recipient should see, rather than hiding parts of your working copy. Hiding is a display setting, and it takes two clicks to undo.