Hi @ListenRelax
Welcome to the Sonos Community!
A couple of answers for you - no, this should not be happening, and no, this is not related to you being on WiFi or ethernet.
Sonos does broadcast WiFi for the initial setup of your devices, though this is only done to connect your phone/tablet in order to teach your Sonos system your WiFi password, and it should turn off once this process has completed. If setup does not complete, the broadcast should turn off after 30 minutes.
First, to verify that it is indeed your system that’s doing this and you’re not seeing the results of a neighbour trying to set up their own system, please unplug all your Sonos devices and refresh the WiFi list on your device. I’m asking you to check this because you mentioned that the broadcast remained after factory resetting your system (don’t do this again, please), so it would make sense for another to show up while setting up the reset devices again. Also, after a reset, the first broadcast should not have remained.
If you verify it is one of your devices doing this, please have each one in turn being the only device powered on until you can determine which device is broadcasting. Then, please submit a support diagnostic and reply here with the number given, and the last 3 digits of the serial number of the device responsible. Thank you.