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 photogra

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.