iFixit has discover something unique about iPhone 4s. Its proximity sensor is working harder than most smartphones (inclusive of other iPhone models). And then, they realized that the culprit might be Siri, the personal assistant.
According to iFixit, Siri can be activated when you raise the phone. In this case, the phone is using the proximity sensor to gauge how far or near the phone it is to an object.
In iPhone 4s, when you turn on the screen, the proximity sensor is already working. In other smartphones (including iPhone 4), it only starts work when a user make a call.
Here is the explanation from iFixit. Today, I learned a new thing. The proximity sensor light can be seen with a webcam. I tried it and you really can see the light. Nice.
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(source)