I'm not familiar with Nest, if you don't have a subscription to store video, and someone rings your doorbell, and you take 5 seconds to bring it up in the app, can you scroll back and see those 5 seconds? Or is it literally a feed from the camera to the app over bluetooth? Probably not. The video stream probably gets sent to some backend system and, while she didn't pay for storage, it probably persists for a few hours to days in cache.

Not set to save recordings likely means "saved in a memory buffer and never given a file handle", that is how most consumer recording devices that offer limited playback work.

I recently had to attempt to piece together dash cam footage from my wife's car in the same way when she witnessed an accident but the file had been "aged out".