I'm experiencing an issue with a MOSS installation. I am using the Site Settings > Permissions to add an Active Directory account as a valid user of a site. This entails validating that the user account name is correct via the 'Check Names' button, then giving them 'Contribute' permissions. Once this is done they appear as a user on the 'All People' page.

This works fine and the user is able to access the site. At some point in the future (sometimes several days later) the user account is somehow removed as a valid user from the site. This site resides in a test environment so access is pretty well controlled; which has allowed us to rule out someone else going in and removing the user manually. This appears to be something that is being done by the system itself and we have no idea why. We can manually add the user back, but then it will eventually get removed again later.

I have an admittedly limited understanding of SharePoint permissions, but I believe that SharePoint stores valid users in a SQL database and I would assume that when dealing with Active Directory accounts it would be storing the user name and probably the SID. It appears that for some reason this record is later getting deleted out of the database, as the users will suddenly disappear from the "All People" page and will start getting "Access Denied: You are not authorized..." messages when trying to access the site.

Has anyone seen this behavior before?