It seems to me that the proximity sensor on other phones is always on too, although I suppose it is possible that some phones only activate it "sometimes". On the G6, at a minimum if you have the AOD enabled, the proximity sensor is being used in order to blank the display when the phone is in your pocket, so it would always be on as long as the screen is locked if you have AOD enabled.
Whether or not you see a visible light from it seems to vary (depending on your eyes and your phone). Pretty sure these use infrared, but most IR LED's in this application will be very close to the visible range, with a broad enough spectrum to be visible to some people (but possibly some manufacturing variance as well).