So, could this be used to find better hiding spots for hidden cameras?
I assume minimizing the aperture would be a fairly reliable way to avoid detection. A pinhole camera would be hard to find by any optical technique, though the video quality would suffer. Perhaps one could project through a pinhole onto a screen and record the projection, so the sensor is angled away from the room?
I think the easier way would be to hide the cameras in much harder-to-reach places so that it's inconvenient for the user to get their smartphone near. This might reduce the kind of videos that can be taken, but maybe an attacker will find that a reasonable tradeoff.
The spy camera could also something like your system to detect the phone camera and take defensive measures.