I've determined that the current Smart Defrag Portable release leaves behind a scheduled task on Windows XP and higher. The job file calls the Updater.exe within Smart Defrag Portable to check for a new version. If one is found, it opens a web page to let the user know. No EXE files are left behind and, if Smart Defrag Portable is no longer on the system, nothing will happen.
This can be solved by adding the following lines to the SmartDefragPortable.ini launcher file in App\AppInfo\Launcher after the DirectoriesMove section:
I'm working up a patch for the app itself now.