While playing via UPnP independent which way (App on smartphone, Volumio website) I can connect with my smartphone to Volumio by Bluetooth.
The playback stops and the playback via Bluetooth starts.
In this case it does not matter, because I did it.
The problem here is, no secure modus prevents from connection, i.e. every in the Bluetooth area can do it and can interrupt my playback.
To prevent this, a self-defined passphrase could stop it.
I suggest a dialog in the website or Volumio App to set it.