SharePoint has a hidden users list which stores the user information. This list exists for every site collection. It is only visible for administrators and there will not be any link directly provided other than typing in the Url.
If a user is deleted from the site, this hidden list still holds the details for that user.
Whenever a user who has access to site collection hits the site, an entry is made in this list. SharePoint stores user information in two different places – user profile store and site collection user list(one for every site collection).
It keeps this information in sync with the help of 2 timer jobs.
- User Profile to SharePoint Full Synchronization
- User Profile to SharePoint Quick Synchronization
If any user is deleted from this hidden list, user will be added back once he/she accesses the site again. If you are looking to delete any user from this hidden user list, use this link which provide you the action to delete user
http://sharepointsite/_layouts/15/people.aspx?MembershipGroupId=0 or browse to any SharePoint group created and replace the Id value to 0.
When i had an issue where a user was not able to access the site because of an update to his account name, I cleared the user from this hidden list and he was able to access the site back again. Reason turned out to be user profile details being out of sync in user profile store and this hidden list in site collection. The moment user hits the site a new and updated entry is made again.