I thought it was established that the doorbell camera was disconnected from the network and that it was a different camera that later registered movement.You're definitely right that these battery-powered wifi cameras will detect stuff even when they are off of their mounts, as long as they're still in wifi range.
However, the problem I have with this theory is that LE have a very specific "disconnected" time for the doorbell camera, and have had that time for a week now (it was on the sheriff timeline in the Thursday press conference, at least). While we've been assuming that when they said "disconnected" they meant that it shut off, was destroyed, or was otherwise disconnected from the network (in which case it would not have been continuing to detect motion and sending alerts), they theoretically could have meant it was physically disconnected from the wall. But the only way I can see them knowing that is if they had pictures or video of it being disconnected from the wall, and they didn't get pictures until this week. Without pictures, they'd know it was removed (because it was obviously gone), but they would not know what time.
Because they didn't have pictures at the time they provided the timestamp of disconnection, IMO the only reasonable explanation is that that's the time the camera disconnected from the network, and it could not have continued detecting after that (ok, it could have reconnected but I feel like they would have mentioned that). So I think the most straightforward conclusion is that the detection was from a different camera. We already know from other statements and from pictures on the scene that there were other cameras; we just don't know what, if anything, the other cameras detected, and which triggered this supposed later notification.
JMO — there's obviously still a ton we don't actually know here, so we're all guessing.
However, my doorbell does indeed have the ability to alert if it is removed from its mount. It can even be set to play loud alarm if it’s tampered with in that manner.
I don’t think that’s what happened here though. I think it’s two different cameras.