Schema Browser filters get saved by the Schema name, so databases which have the same schema name will use the same filters.
User has a schema ABC on database PROD, ABC on database TEST, and ABC on database DEV. If they log into ABC@PROD with Toad, go to Schema Browser, and create a Filter, the filter gets saved in users file as ABC.FLT. However, when the user then logs into ABC@TEST or ABC@PROD, it will also automatically use the same filter ABC.FLT for this database and schema.
This issue has been reported to the developers and is waiting for fix in a future release of Toad for Oracle.