Comment by fortran77

When I worked for a big Hollywood media conglomerate, there was a project to detect cameras in theaters. (There was a piracy problem where people would record the movie on a camcorder). It worked by detecting the IR filter that’s in front of the CMOS detector in almost all cameras. It’s a retroreflector for UV range. Shine a UV light to the audience and look for spots of light. I’d imagine this would work for cameras in any darkened environment even today.