Ownership changed without approval

I cannot be sure what has happened here, but transfer of registration of devices does not require access to your account. What it requires is for the speakers to be added to somebody else's system. Logging into any account isn't going to transfer speakers.

The simple act of connecting a speaker to the same wireless LAN would not normally cause any transfer of speakers - the two systems could and should remain separate. So it begs the question whether something else was inadvertently done, with unexpected consequences.

Perhaps a Sonos staff member or other Community member will be able to shed more light. It is perfectly possible that with recent changes to speaker registration I have missed something.

I suspect what may have happened here is that the son's Sonos was originally part of the main house system (i.e. it shared the same Household ID).

The player was taken away to another dwelling. Various changes were made whilst it was detached, including registering it with a different account.

When the player rejoined the main system the latest account changes would have been those on the returning device. Presumably these then propagated to the rest of the devices in the system.That seems very plausible. The return of a prodigal son (in a Sonos speaker sense) is always going to be a bit unpredictable (to me, at least). It would be interesting to know what has become of playlists and favourites, given that they will likely have diverged during the period of separation.

If this is indeed the explanation, it would have been better if the son had factory reset after taking the speaker, so that it registered to a different household ID. It would then have behaved in the way I said it 'should' have done, remaining separate.

If you factory reset the device now you will wipe all the playlists (although recoverable by adding it back to the main system). You might do better to leave them alone as one system if your son cares about those playlists etc. He can take it away with him at some point then never bring it back! Do take care with factory resets - you need to understand the consequences not just see what happens!