Special USB Port With "Sensor" Contact
[ I haven't come across a USB jack with a cable detection mechanism, until now.
So, the following is just a hypothesis. ]

This additional pin provides a mechanism for detecting if a cable is plugged into the jack, even if there is nothing at the end of the cable.  Have a look at the pinout for the standard USB type-A jack.  Notice that the sensor pin sits just above the standard GND pin.  When a cable is plugged into the jack, it flexes the GND contact which touches the sensor pin.  Essentially, it's a switch.

You can test easily this hypothesis.  


Check the continuity between the sense pin and the GND with a multimeter.
It should be an open circuit.
Then plug in a USB cable and check the continuity between GND and the sense pin again.
Now it should be a closed circuit.