I've seen this twice in the past few days. The computer boots to a 4 digit input and the indication to write
applepass04@gmail.com. One person did this, they responded with a bitcoin wallet ID and a ransom of 0.03085 bitcoin, which equates to $50 USD. A firmware password does get set, so the only option one has is to take (original) proof of purchase and the Mac to the nearest Apple Store, and the Genius Bar can, after POP verification, remove the firmware password. Not sure what hurdles to jump after that to possibly recover data, unless you're ok with paying a ransom for your data. At the very least, when the firmware is unlocked, you can at bare minimum, format and recover the device if not your data. This is a bit disconcerting to see. I've heard of large corporations data being held prisoner for large ransoms, but it's worrisome to see it filtering down to personal computers, Macs at that, which is surprising because macOS is a pretty secure environment, especially if you're fully up to date with the latest version.
I suspect iCloud might play a role, because one of the two instances I've seen had two Macs, and an iPhone hit at the same time.