If, after hiding a worksheet, you use Tools>Protection>Protect Workbook, the hidden worksheets cannot be unhidden via Format>Sheet>Unhide - you first have to Unprotect Workbook in order to show them. If you add a password to Protect Workbooks, this makes it even more secure.
If you have macros that need to show or use the hidden worksheets, you can incorporate an Unprotect Workbook line at the beginning of the macro and Protect Workbook line at the end.