Something odd going on! I don’t have a Alexa so can’t get my head round this!
Guest in our holiday let has brought her Alexa. She’s connected to our WiFi, nothing else, to play music. But it’s playing out of next door’s speaker system which is hardwired in the ceilings apparently. I can see that their speaker network is unsecured so I’m guessing that Alexa has searched for a speaker and found that one?