Steve nailed it.

However, I'm not sure if this would be applicable in your environment, but we had one user who used their login credentials in an excel spreadsheet and this was still active after he left.

We're only a small company, so I know who is here and who has since left...For a company with hundreds or thousands of users, you might well cross reference your login list against an HR employee list to see if you have any applications using logins which need removing.