Toad expects users to be able to select from USER_ and ALL_ views because they are granted to public, however you can prevent these queries in both versions by unchecking this option:
View | Options | Schema Browser | Data Tab | Highlight columns populated by sequence/trigger pair
If the error appears when selecting the Triggers tab, there is no workaround for this at this time, as Toad by design needs access to the ALL_SOURCE table.
STATUS:
For the Triggers tab scenario, enhancement request TOR-5184 has been submitted to Development for consideration in a future release of Toad for Oracle.