SQL Server db user mapped to windows login not in SQL Server logins

Question

I am on a SQL Server 2008 where I login with DOMAIN\USER_A

On my database I can see this login is mapped to the user dbo of that database. What I don't understand is that DOMAIN\USER_A is nowhere to be found in the SQL Servers Security Logins.

CURRENT_USER reports dbo SYSTEM_USER reports DOMAIN\USER_A as do SUSER_NAME

I can't find DOMAIN\USER_A in the sys.syslogins? What is going on? Can a database user have its own login circumventing the SQL Server?