For now, I will just mention the four Dots and one Echo, and if necessary I will mention the Taps at another time.
Was checking the four Dots bluetooth settings. First, it helps if you are SURE you are working on the Dot you intend to. Alexa app shows you serial numbers of all Dots and numbers them, but when setting them up you only see the last three characters of serial number. Fine, so don't get mixed up when working on 3rd Dot when you intended oto be working on 4th Dot. No wonder app showed I had paired the Dot, but it was not working; it was the Dot upstairs and I was checking app for a downstairs Dot! Got that figured out eventually, and all was well.
During this futzing, I noticed my 3rd Dot showed that my new next door neighbor's iPhone was one of the Dot's Bluetooth connections! How in the heck did that happen? Fortunately, I could delete/remove his iPhone from the list. My wife's iPad's BT was also connected, too, and I deleted that, since it was unintentional.
Well, I think it can happen because some people obviously never turn off BT in their phones and tablets. Some people have the BT setting of their device to auto-connect to any detected BT signal; not good either. And, now it seems an unauthorized BT connection can be made on an Echo device, but will it work? Maybe so. Perhaps someone can get a friend to connect his phone BT to your echo device and see if it allows for him to send music to your Echo. I sure hope not. (His phone can be deleted from the Echo's BT connection list afterward.)
Who wants to try this before I get a chance to and report back here
?