Somewhere in my HN comment history from a while back is a response to a person claiming that modern phone cameras can’t detect IR illumination and remotes.
I took a bunch of modern iPhones and Android phones, from colleagues in an IT dept, and demonstrated they can in fact see a bunch of different IR remotes and illuminators with the rear camera.
I could find zero cameras that could not see the IR.
I’m not sure where people got the notion they couldn’t.
The ultimate answer is "it depends". And upon what it depends is the particular IR wavelength the camera emits to "illuminate" a scene in IR for night photography.
My cell phone's back camera will show IR light from IR remote controls (I've used it for just that to verify that a remote is transmitting). But I also have an outdoor IP camera with IR illumination in my back yard. The same cell phone camera sees zero IR emitted from the outdoor IP camera (even though it quite well lights up a fairly broad area of the yard at night).
So for my phone, if a 'spy cam' were using the IR wavelength the IP camera uses, I would never know it was present by using the phone camera. If it used something closer to the wavelength used by IR remotes, yes, then the 'spy cam' would light up via the phone camera.
"I’m not sure where people got the notion they couldn’t."
I make IR devices. My phone is the only one in the warehouse that can pick up their emissions - everyone else's cameras have IR filters with what appears to be a sharp ~750nm cutoff. I'm the only one that will pick up 800-1064nm with my cheap Samsung, and so I'm the only one doing the testing on those diode assemblies.
Ok, I'll admit that I have not tested that many phones, but all the phones I had in the last ~8 years would not detect my remotes unless I used the front camera. That could very well be because all the remotes I tested operate on 940nm instead of 850nm (the two common options), and the IR filter in my devices are so that it would cut the one but not the other. Or that they just have a much lower power level. Either way, most modern phones that double as cameras will have an IR cut filter of some sort, otherwise some photos appear weird - like the red glow from a fire will appear purplish-white.
I don't but it's easy to test, just pick up a TV remote and press a button while pointing it towards the camera. It should look like a flashing white LED.
I should also mention that both IR illuminators and TV remotes are usually either 850nm or 940nm, I have not looked into that aspect of it. I imagine that it's possible that your camera can detect one but not the other...
Easy test - check your black levels against someone wearing a black fabric or a reflective black surface. If you get a fair bit of grey in the image, odds are very great that you don't have an IR filter installed.
Most of the bright IR lights you find on typical surveillance cameras actually visibly look a little red. I wouldn't think the same lights would be used if the intention is to spy in secret.
Would this also be able to detect something like a camera that saves videos to an SD card to be retrieved later? Something that doesn't use WiFi or a radio?
That's the main limit I see, but I'm wasn't sure if it such a device would still generate enough RF intrinsically w/o a radio.
The phone camera will pick up the bright IR lights that hidden cameras use to illuminate the room-- wireless or not.
Obviously this only works if the camera uses IR lights, but pretty much all of the sneaky ones do.