Proximity sensors typically use infrared. There's two main components: the IR light source (an LED), and a sensor that measure how much of the IR light is bounced back. The human eye can't see IR light, but depending on the wavelength used, there could be SOME portion of the light that barely falls within human vision on the red end. That could account for the faint red glow.
What this would imply is that the proximity sensor is on when you see the red glow. If it's frequently on and you're not in a call or using an app that requires the proximity sensor, then something is messed up.
The good thing about science is that it's true whether or not you believe in it. -Neil deGrasse Tyson
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