If you follow the ‘not connected’ link showing on the device what do you see?

I’ll check when I get home and post then. 

If you follow the ‘Not connected’ link until you see the option to ‘Hide your product’ then the Sub will be hidden until it is connected back to power.

I returned the sub as I really don’t need it . Just kept the Arc. 

It shows the following. I know longer have the Sub . Hiding it is an option, but doesn’t actually delete it. 

There is no way to actually remove devices in the Sonos ecosystem - that will happen automatically. Just hide it and if you want to be certain the device has gone, power off all your devices and then bring them back online … (Personally speaking, I wouldn’t bother) … Hiding it means you’ll not see it again.

The ‘Not connected” is a recent additional “feature” of the app.

Previously, if you had speakers in your system that were not powered on, they would still appear on the Sonos network matrix. Disconnecting power from all Sonos speakers, then rebooting the router and powering on the Sonos speakers would result in only the connected/active speakers showing in the matrix, and consequently the app.

I assume such action would result in the same outcome, but not 100% sure.

Hiding the sub is the easiest/quickest solution and I’d imagine at some point the hidden speaker would eventually be removed altogether. Potentially at the next Sonos update when all players reboot.

edit: I took too long pondering what to write, Ken beat me to it.