Yes, I am now pretty sure it is Tunein’s widget that is popping up the advert. Although Tunein is installed I never launch it. I thought, incorrectly, that I needed Tunein installed in order to use it through Sonos. My solution to those messages was to locate the Tunein app in the running app list and force stop it. That *used* to work great on Android, and a force-stopped app would STAY force-stopped, but sometime in the past year an Android update must have overridden that as after every restart of my mobile I find the force-stopped apps have restarted.
Also, based on Corry P’s comments above I uninstalled Tunein from my mobile and Tunein is still a working music selection on my Android. Even after an Android power off/on.
Thanks all!