OS X Mavericks includes a new feature that leverages the light sensors included in many Macs to detect movement in front of the machine



From MacRumors:

First highlighted by Moshen Chen of Radiantlabs and confirmed by developer Jonathan Wight, the feature was initially thought to use the iSight camera to monitor movements but was quickly discovered to actually be tapping into light sensors.The sensor is already used to adjust screen brightness to ambient light, but the new OS puts it to a different purpose, tracking changes in the light as "movement," and resetting idle time accordingly. Verge tests confirmed this on two separate Mavericks laptops: after covering the camera but not the light sensor, we were able to delay sleep mode by changing the ambient lighting cond

  OS X Mavericks Taps Ambient Light Sensors to Detect Motion and Prevent System Sleep