updated 03:53 pm EDT, Fri November 1, 2013
Mavericks uses light sensors as motion detectors
Apple's newest Mac operating system, OS X Mavericks, uses the light sensor included in many Macs to keep a device from prematurely falling asleep. The functionality was discovered by by a coder looking through Mavericks' code, and the coder thought that one element indicated that the iSight camera is always on and watching the user. Instead, subsequent investigations revealed that the OS taps into the ambient light sensor.
Tests pointed out by The Verge showed that the light sensor to the left of the iSight camera - which monitors conditions in order to adjust screen brightness - can also be used to track changes in light that are caused by user movement. The sensor then resets the system's idle time.
In effect, Apple appears to have built in the capability for a system to know whether its user is sitting in front of it, even if the user is not doing anything. Such capability has the effect of keeping a computer from going to sleep if a user is distracted. It can also, though, apparently keep a device awake just due to changes in ambient lighting.