I was going to say it's pretty unusual to have a specific account permission on the root of your drive, but I have the same thing. The full SID:
Code:
Account Unknown(S-1-15-3-65536-1888954469-739942743-1668119174-2468466756-4239452838-1296943325-355587736-700089176)
Results on Google suggest it is a security thing Windows is doing related to app containerisation, so a permission for giving particular applications restricted access into the filesystem.
They are app capability SIDs.
devblogs.microsoft.com