Can a PIR sensor be covered by anything at all or does any kind of material automatically block motion detection?
Technically the sensor is covered by something, but that thing is the focusing Fresnel lens. Depending on the PIR you got, you’ll notice the plastic domed lens does come off rather easily.
“These curved segments are Fresnel lenses which constitute an array that increases the detection zone of the sensor. Fresnel lens array is known to capture more infrared radiation and focus it to a relatively smaller point. Detection is more stable and maximum distance for detection is also increased. Fresnel lens has been crafted to be translucent so that it can capture only infrared radiation without getting unwanted radiations from visible spectrum of light.”
Now the issue is that most glass and metals reflect IR wavelengths extremely well. So depending what the film/mirror backing is made out of may make your efforts mute. Aluminum which is used in most standard mirrors reflects all of the Infrared light in the spectrum.
So mounting it behind the mirror just doesn’t seem practical due to the way light works. So what I’m thinking of for me, is to mount it in a light fixture above the mirror. Not sure on the details yet but that’s where I am right now.
However the Camera trick does function very well and will give you that clean look.