AD User/IP Displays legacy users - AD Updates?

unknown

So I have noticed that our L1 team have been joining corporate users to our system using their own credentials (eg. ptom/172.20.20.1).
Some of these L1 techs have since left, however they remain authenticated on the system. Does the ZD1100 constantly refer back to AD to update active users? 
Or is it that once authenticated on the WLAN, they remain so indefinitely.
From an auditing point of view this seems like an issue on tracking staff logins.