Running a script with task manager using the SYSTEM user the environment variables of this user (on Windows 10) are:
APPDATA: C:\Windows\system32\config\systemprofile\AppData\Roaming
LOCALAPPDATA: C:\Windows\system32\config\systemprofile\AppData\Local
HOMEPATH: <<<EMPTY>>>
Path: <<<SYSTEM_PATH>>>;C:\Windows\system32\config\systemprofile\AppData\Local\Microsoft\WindowsApps
TEMP: C:\Windows\TEMP
TMP: C:\Windows\TEMP
USERNAME: <<<PC-HOSTNAME>>>$
USERPROFILE: C:\Windows\system32\config\systemprofile
Notes:
HOMEPATH is empty instead of \Users\John for an user named John
USERNAME is the hostname followed by a dollar sign