Very true. But whereas I can see my neighbours routers (if their SSID is displayed on), I can't see each individual device connected to them. To do that I would have to be logged in to the router itself.
Simple way round though, just change the SSID and password. And then set up access i.e. only the allowed mac numbers (the OP devices) to connect. Sorted.
Of course it doesn't explain why the problem is happening in the first place - unless BT only have a couple of default passwords....one reason to ALWAYS change them from the default.
As for the Microsoft password sharing feature with Windows 10...words fail me.