Sonos has a FAQ in the support section on blink codes.
https://support.sonos.com/en-us/article/sonos-led-lights
Fast white is booting, it may be seeing an internal issue. Power it down for a minute and then back up, if it doesn't clear up call support.
Thanks for the response @Stanley_4.
It's blinking really fast, like 5 times per second. Not really sure if this would count as the blink code for booting up, but most probable I guess. My second zp90 is showing the same problem/blink code.
Why can't I find it using the instructions in the S1 app?
I'll see what other people post here and if I can't figure it out, I'll call support after the weekend.
As long as they are wired to Ethernet the fast-white is the booting status.
If the long power-down didn't help support is your best option. Many folks panic and do a factory reset, a bad idea as it wipes diagnostic data and any surviving configuration making support more difficult.
Until they boot the app won't see them, you might see them on your network, maybe even access the internal web server but that won't do much for you.