When initializing a remote PowerShell, a "tmp_ <a UID>" file is created in the Temp Job Service user directory which contains 3 files (a total of about 3 MB). Upon completion of the session, this directory is removed.
Some Exchange jobs may cause only one of two of these directories to be removed and can leave a tmp directory in the user profile directory under %temp%, even if the "Remove-PSSession $session" - script has been executed correctly.