The proximity sensor in the iPhone 4S is active all the time the screen is active. The reason for this is the “Raise to speak” setting on Siri that can detect when you lift your 4S to your head so it can activate Siri.
On all other iPhone models (iPhone 4, 3GS, 3G) that have a proximity sensor, that particular sensor is only active when making a call so that it can detect when the phone is touching your head and can switch off the screen and touch features to save battery and prevent you muting calls etc… As Siri requires to know if it’s touching your head at all times the screen is on, it is always active. That is unless you disable that function of course.
To give a run down of what the sensor looks like and how it looks while being active take a look at the iFixit video below which uses a webcam to capture the IR being emitted from the sensor. Note that this kind of light isn’t harmful.
Source 1,2
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