Created a synonym through a db link on an object that is not owned by the current user login. Schema Browser does not display the synonym even with the "Through DB Link" filter option.
Schema Browser | Synonym tab queries All_objects view in addition to All_synonym view. Objects not owned by the user will not be included in the All_Objects view.
Issue fixed in version 9.0.1. The latest version of Toad for Oracle can be downloaded here.