I think I found what may be causing this prompt for a read-only file
(re-discovered by reviewing another thread)!

In the SaveAs dialog box, click on the Tools button, then select the "Save
Options" command. The Save Options dialog box has the following options
(Excel 2000):

Always create backup (checkbox)

File sharing
Password to open: ________
Password to modify: ________

Read-only recommended (checkbox)

If you enter a password to open, Excel will prompt for a password when
opening the file.

If you check "Read-only recommended", and then open the file using the
password, Excel will prompt with "'Example.xls' should be opened as
read-only unless you need to save changes to it. Open as read-only?". The
message box has Yes, No, and Cancel buttons.
--
Regards,
Bill Renaud