The coins in your address should be "mostly" visible to Mint - your address is still a public address even if it's hostd at Coinbase. However, that said, any internal Coinbase-to-Coinbase transfers don't hit the public blockchain (for a number of reasons) so if you have a lot of those then your balance would be incorrect. Any transfers from outside of Coinbase to your address or from your address to outside of Coinbase would be visible on the public blockchain.