Objects that you set into the session using setAttribute are known as
scoped variables. You can also set such variables into page (if a
JSP), request, and application scopes.
For the session, I do not believe that there are any built-in values. So if you don't put anything there, there's nothing to retrieve.
The container will sometimes populate the other scopes with some built-in values (for example, putting error information into request scoped variables when an error is detected).