Two kinds of hidden
A normally hidden sheet can be restored by right-clicking any tab and choosing Unhide. A very hidden sheet does not appear in that list and needs the developer tools to restore, which stops a casual reader but nobody who is looking. Neither is protection.
What tends to be on them
The working. Cost bases, supplier rates, margin calculations, the assumptions behind a headline number, staff details, an older version of the model. These get hidden rather than deleted because the visible sheets still depend on them.
The right fix
Build a fresh workbook containing only what the recipient should have, with values pasted in rather than formulas pointing at sheets you removed. Sending a hidden copy of your own working is a habit worth breaking.