Eventually the will disappear from the app on their own, and they will be removed from your account when the new owner adds them into their system. 

Hi @jgatie thanks for responding, appreciated. Hope you’re well

This has to be then a SONOS app/software issue, potentially a wider issue...

It does not make sense that you have to wait for a new owner to set up speaker, and only then the speaker (factory reset from own account) can be removed I assume automatically from own S2 app??

Is this an own account thing.. surely SONOS system and process cant mandate that any speaker you remove from your own system, cant be removed from S2 app until someone else owns and registers that speaker !!!!? Ouch!

Is there a data protection issue here, as removed speaker is still linked to your own account/system and app, even after you remove from system and factory reset… the removed speaker is still linked to your account and app when you pass onto family, friend or a local sale. That cant be right?  

Factory reset, should mean exactly that. Reset speaker and removed from any system, settings, preferences, config and apps….

Does anyone, SONOS team, SONOS community have any information/way forward with this issue

Many thanks 

You misunderstood.  Regardless if you reset the speaker and/or it is registered elsewhere, removing it from your network by simply turning it off or taking it out of range will result in it disappearing from your app in a couple days.  This is to keep devices which are removed temporarily, like to bring to a friend's place, from inconveniently disappearing just because it was off your network for a couple hours.

This has nothing to do with removal from your account ownership, which is the list of devices you own under your account here at Sonos.com.  Devices only disappear from that list if they are registered elsewhere.  Otherwise, your devices would disappear every time you factory reset, and there are users who think (wrongly) that a factory reset is something to be done at the slightest sign of a problem.  If you think they should disappear at the very instant they are reset, then you and I both should have a word with the chronic resetters.  Unfortunately, history says that will do no good. 

Hi @jgatie many thanks for y9ur time, help and info, appreciated 

I was about to post a question on this.

I recently stopped using a Play:1, 2 x Play:3 and a Sonos One.

l see them in the app after doing a factory reset 2 days ago.

I still have four speakers in use so it is frustrating you cannot just delete or hide the unused speakers.

I will wait patiently...

You can hide them in the app. Just follow the prompts.

Or wait a few more days and they should age out. 

I could not find a way to hide speakers.

However, and it may be a coincidence, I grouped a couple of the working speakers this morning and when I looked at the app again later the offline speakers were gone.

So either they aged out or the grouping forced some kind of refresh.

Anyway, happy now

The offline speakers oddly reappeared  however I did find out how to hide speakers.

Settings->System

Select offline speaker

There is an option to hide the speaker