Yeah, that's fair. If someone really wanted to do this though, they could also just use existing products like this one: https://www.amazon.com/Worldoor-Multi-Detector-Full-Range-Al....

Maybe, but realistically, since the sensor uses the properties of a camera lens to detect the camera itself, I would have to bet there is little that could actually be done to obscure the hidden camera and also still get a usable image from it. This is a pretty brilliant detection method.

It might be easier to design the room to create false positives and undermine trust in the detectors. Assuming you have that kind of influence, anyway. That's not an option if you're just trying to plant a camera in an existing room.

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?

That's an interesting idea: trying to create so many false positives that the user gives up. We're already removing around 100+ false positives per frame at the moment in difficult cases, so perhaps it's possible to overwhelm the filters with very maliciously designed environments.

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.

Probably the very smart spy camera can detect off axis TOF artificial light and close a shutter over the lens. Assuming your app integrates over the scanning motion, the camera won't be detected while it's shutter is closed. After the illumination is completed the spy camera can re-open it's shutter.

The spy camera could also something like your system to detect the phone camera and take defensive measures.

Yes, perhaps. At the very least, adding a variable shutter and extra logic will drive up the price of the camera, which will be some consolation.

A honeycomb lens filter (commonly used in military rifle optics) would greatly reduce the angle that the camera can be detected.

what about fiber-optics to direct light from a pin-hole? would the entrance of the fiber-optics cable be detectable this way?

absolutely, at least against this method of detection

Security is always an arms race. Making the person hiding the cameras make the extra effort is worth it because some will be too lazy. Also some wont know and be caught which increases precieved risk which may tip the risk-reward scale more towards risk for the person hiding the camera making it less likely they would decide to try.