A patent filing from Apple shows a system that can detect when your iPhone is smudged, and stop the display from turning off like it does when you hold your iPhone near your ear.
When the iPhone screen has a smudge from your finger, for example, it blocks the path of light back to the phone's sensors. When that happens, your iPhone might think that you are holding your phone up to your head, and then turn off the screen. Or it could think that you're not holding your phone up, when in fact you are.
The patent outlines a system that measures the light coming onto the iPhone screen to understand whether the obstruction is the result of being held up to your head or from something else.
Here's a diagram from Apple's patent which explains the problem. The section marked 132 is a smudge on the iPhone screen:
Apple is so keen to make sure that the rare instance of a smudge accidentally turning off an iPhone screen doesn't happen that it has patented a system that can detect smudges, and change the light pattern of the sensors to get around it. It can tell the difference between two types of smudges, one that blocks light and reflects it back, and one that lets it out when it shouldn't.
Here's the other kind of smudge. This one lets light out when it shouldn't:
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