I have added two Move speakers to my Sonos setup. Unfortunately, they keep disappearing from the System list. For instance, right now, my "Office" Move speaker is missing from my iPad app, iPhone app, and pc (although it is available on my Android app). My "Kitchen" Move speaker is missing from all four. When I check again later, I expect both speakers to be on the list, without me having done anything. They keep popping in and out of existence. (Also, sometimes, the devices will show up on the list, but I won't be able to control them: Starting playback fails with an error message stating that, well, playback failed.)
I've read the Standby thread (https://en.community.sonos.com/wireless-speakers-228992/move-standby-question-6830346) with interest, but I don't think that's the issue here. The speakers are not on standby. For instance, right now, the "Kitchen" speaker reacts when I press, say, the Microphone on/off button, and the "Office" speaker will even resume playback of the podcast I was playing yesterday when I press the play/pause button on the device. Restarting the apps and/or speakers doesn't help. Waiting does. (Update 30 minutes later: Sure enough, both my "Office" and "Kitchen" Move speakers are now magically back on the list, and working well. I haven't touched a thing.)
The rest of my Sonos S2 setup consists of Play:1s, Play:3s, Play:5s and a Playbar running off a Boost. Apart from the Move issue, the system is rock solid. My router stability and wi-fi coverage (which I understand the Moves may be using) is excellent.
Any idea what's going on? If you need a diagnostic number, don't fret. I'm sure one or both speakers will disappear again in the near future.