I would suggest that this is simply an unknown account. It will comprise of the letter S followed by the actual ID reference.
There are various reasons for this .....
As NTFS permissions will survive a re-installation of the OS this may be a remnant from a previous OS or install.
It could also be a user that still has access due to dual boot access from another windows OS.
You can remove this if it is a remnant but would not advise this if it is from a dual boot
To remove the ID you may have to navigate up the folder tree to the folder where the permissions were initially applied, as inherited permissions are more complex to remove
The following is an example of such an entry (from a dual boot Vista install in this case)