An offline version of Windows Defender has been spotted in the Windows 10 Insider Preview. If you're rocking build 14271, it's possible to reboot the installation into a more secure mode to begin scanning the PC for threats to be removed without requiring the program to be on external storage.
There'll be a new option in the Settings app (under Security + Updates > Windows Defender) that will allow you to select "Scan Offline". This will cause the system to reboot and is estimated to take around 15 minutes to complete. It's a neat feature for those who choose Defender as their main line of protection.