According to a few users, the ~BT Windows 10 folder has been appearing on users' machines even without them agreeing to the upgrade from their Windows 7 or Windows 8 installations.
Microsoft has confirmed the news with The Inquirer, VentureBeat and PCWorld. PCGamer reports: "Users have been able to opt-in to the free Windows 10 upgrade if they're running Windows 7 or 8, but apparently even if they don't, the files are being downloaded anyway. It's happening on devices which have automatic updates allowed, which we'd generally recommend to catch the latest security and stability updates for Windows".
Microsoft sent a response, where they said: "For individuals who have chosen to receive automatic updates through Windows Update, we help upgradable devices get ready for Windows 10 by downloading the files they'll need if they decide to upgrade. When the upgrade is ready, the customer will be prompted to install Windows 10 on the device".
This isn't good for people with slower Internet connections, or smaller download caps as the download of Windows 10 falls between 3.5GB and 6GB.