If you have automatic updates turned on in Windows 7 or 8.1, your OS will quietly download Windows 10 for you, weather you chose to reserve your Windows 10 free upgrade or not. It's not yet confirmed if it's a bug or Microsoft is doing this intentionally. The only way to stop it, for now, is to turn automatic updates off and update your Windows manually.
If there's a hidden ‘$Windows.~BT’ folder in the root directory of your main hard drive (usually C: drive), it means Microsoft has already started downloading Windows 10 for you. The download will be between 3.5GB to 6GB of data.
Source: The Inquirer