Word 2002 Windows XP

One of my users has a Password Protected document that she says she did not
password protect. We have a doc management system, and when I pull up the
copy of the last person to use it, it was not protected.

The only way I know to password protect a document is to go to Tools,
Security, Protect Document, type a password, click OK, retype the password,
and click OK again. The user says she would remember doing that (and I
agree), so I'm inclined to believe that she didn't do it.

Other than someone walking into her office and password protecting a
document that was on her screen, is there another way documents can become
protected? Could it be due to corruption? (I'm not looking for a way to
UNprotect the document, I'm trying to figure out how this happened, so it
won't happen in the future.)

Any help would be greatly appreciated.

TIA,

Doreen