What section is this Property/Security section? I'm not at all clear on what you found.
It may be that this is some sort of .Net Framework account what Windows sometimes create, in which case it can safely be ignored.
Whenever i am downloading any file or creating any file/folder then an unknown account (Account unknown(S-1-5-5-0-649118) is created in the property/security section.
Please let me know if anyone is having idea how to get rid of this ?
Thanks for your time and help.