The smart lock feature on Android allows someone to set a Bluetooth device as "trusted" which makes it easy to unlock. See for example: https://support.google.com/nexus/answer/6093922
I tried to add an Avantree Roxa device to play with this, but the log states:
I/Coffee - BluetoothTrustlet( 1549): Device Avantree Roxa is not allowed because it is either insecure or of unknown security and needs user confirmation
What makes a device secure or not unknown anymore? I have paired with this device multiple times to stream audio. There doesn't seem to be a place where "user confirmation" is possible. Anyone knows how this is implemented?