Don't they have a battery backup and a local buffer before uploading?
It probably had its last footage still stored locally, using the remnants of power in its internal battery.
There's no reason they, however, won't still derive value from it without a subscription by recording and reselling that data somehow. That's probably how they got this footage. All the subscription does it help subsidize their surveillance network and let you use it a little bit.
A lot of these cameras don't store anything locally unless you add an SD card, which die all the time.
There are some limits of course but they are mostly technological, but this ain't some notification trickle but full pictures when you expect zero, zilch, nothing.
I'd never install such device at home, added value is dubious at best for my family life and this is exactly the type of shit I would expect to be happening in it, regardless of brand or country of origin. If it connects it sends. Its sad state of things in 2026 but thats reality right now.
More broadly, if it connects, it will serve other masters besides you.
Nest cameras upload event footage even without a subscription.
It's not a secret. It's a selling point for the devices.