The NTFS file system use a central "Master File Table" for storing the sectors where each files starts and which sectors belong to the file. This Master File Table is stored on the drive in form of an invisible file (
It is crucial for the whole drive that this $MFT file has free space to store infors for new files, therefore Windows reserves some megabytes - this is most likely one part of the used space you see. Therefore event if you format a drive the now complete empty device will again have some used space because the $MFT file is there.
Furthermore there are other hidden files and folders, such as
System Volume Information (contains the volume shadow copy files),
hiberfil.sys and more.
For more details on the MFT see the Microsoft article "How NTFS reserves space for its Master File Table (MFT)"