I believe this device is sensitive to people’s retina- the part that gets lit up red in flash photos.
If a person is looking through a telescope or binoculars, you will see a very magnified image of that person’s pupil if you look in the large end.
So this device apparently projects an invisible wavelength of light and detects the “red-eye” reflection which will be magnified if some sort of scope is being used.
Supposedly it detects camera lenses, which won’t have a retina showing except SLR.