I have a schema where only a single DB_Link exists (SCHEMA1) on the entire schema. On another schema with DBA rights, I open the Schema Browser and go into a Treeview mode. By default I see the SCHEMA1 schema in the "Other Schemas" section. However I then right-click the Other Schema branch and select -> Users to Load | Load Only Users that Own Objects. Then the SCHEMA1 schema disappears. So Toad hides schemas where only a DB_Link exists. (No tables, procs, etc.).
Create an object under the new user created, e.g. a view, then refresh the Schema Browser to check if it is now there. Drop the view if necessary.
Waiting for fix in a future release of Toad for Oracle.