Frank wants to know if it's possible more tunnels exists.
He also thinks the Banished/Exiled need to start talking about what it is like to be a witch/wizard in Muggle society. It's not just 'we can't do magic' since it's the erasure of identity. Frank thinks on some level, quite a few of the younger ones in the Wizarding world will understand that, being forced to not have their own identities either.
He says it's just a thought, but it's better than talking about day to day stuff, or attacking the wizarding world for it's harsh and repressive views.