After removing the S1, we go back to the future, and the display panel, to find a lonesome chip:
Analog Devices AD7166 ARM Cortex M3-based Touchscreen Controller
With some careful tweezing, we pull up what appears to be an ambient light sensor.
We're betting that this is one of Apple's new solar cell ambient light sensors. This allows for the sensor to be behind the display panel, as opposed to the traditional surface-mounted design seen on most smartphones and tablets.