Just saw this recently when using a loginless user using EXECUTE AS. Could be that this is what is happening.

You can verify using this:

CREATE USER test WITHOUT LOGIN;

GO

EXECUTE AS USER = 'test';

SELECT * FROM sys.all_columns AS AC;

WAITFOR DELAY '00:00:30';

REVERT;

And run this in a another query window:

SELECT original_login_name, *

FROM sys.dm_exec_sessions