Even with 'Exclude background Toad sessions' unchecked I still see an extra Toad session from the Session Browser.
There are two features that could cause a new session to be opened in Toad.
1. Enabling 'Execute queries in threads'
To disable, go to View | Toad Options | Oracle Transactions and uncheck 'Execute queries in threads'
2. Enabling a separate session for the Explain Plan
To disable it go to View | Toad Options | Oracle | Transaction and choose “Main Toad Session” under