Re: not scanning your device, exactly
"before an image is stored in iCloud Photos, an on-device matching process is performed for that image"
Are you a WhatsApp user with an iPhone? If so, from time to time you may receive photos from anyone able to look you up in WhatsApp. Those photos are stored on your camera roll right now. Go and have a look. In the default configuration your camera roll is synced to iCloud for the last 100 or so images. (You can disable this feature in WhatsApp > Settings > Chats).
Now have another think about the implications.
Biting the hand that feeds IT
