1. Web Authentication Layer is enabled with IWA realm

2.  Management Console > Statistics > Authentication > Display by user
    The domain\ComputerName$ is authenticated and cached in ProxySG.

3. After administrator manually logs out the domain\ComputerName$, the user can access the Internet 
    Management Console > Statistics > Authentication > Display by user 
    The domain\username or Anonymous users are authenticated and cached in ProxySG.

This is a known issue unique to ProxySG deployments where IP-based authentication surrogate modes are used with IWA.

For the possible root causes, refer to Intermittent Internet access problems when running Windows 7 or Windows Vista through the ProxySG

Common Solution is to implement Content Policy Language (CPL) to logout the ComputerName appended with $, and anonymous users.

<Proxy>
condition=SILENT_USERS realm=<REALM_NAME> user.login.log_out(true) 

define condition SILENT_USERS
  user="NT AUTHORITY\ANONYMOUS LOGON" 
  user="Anonymous logon" 
  user="NT AUTHORITY\Anonymous logon" 
  user.regex=".+\$$"
end