user/login name on process unknown
EXECUTE AS USER = 'SomeUser'; where 'SomeUser' is a user who doesn't have a linked login at the server level will result in SQL Server reporting the login name as a SID starting with s-1-9-3.  

You can use the original_login_name column in sys.dm_exec_sessions to determine who or what is impersonating the user without a login; for instance:

SELECT SessionID = des.session_id
    , LoginName = des.login_name
    , OriginalLoginName = des.original_login_name
FROM sys.dm_exec_sessions des
WHERE des.is_user_process = 1;


I wrote a blog-post over at SQL Server Science showing this behavior, and how to identify which database-level-principal is being impersonated.