Assessing whether or not an "off" device is truly "off" isn't trivial. That's a big part of my point.

I do suspect that running a camera would generate sufficient power draw that you'd tend to notice, though hibernating and taking periodic snapshots might be much harder to detect.

For the casual user, distinguishing a down-disabled device from one that's got a slight charge trickling through it, especially if it happens to be connected to a charger, is going to be easy to get wrong.