How does a non contact voltage tester work?
they work by capacitive sensing of the AC voltage on the live conductor. they only work with AC. 

It's obviously responding to some varying signal on the USB cable, (possibly "ground bounce" due to the varying current draw of the switching regulator in the phone. (USB cables are usually well shielded so that's about all that's likely to be there.

As the pickup is capacitive, the higher the frequency the more sensitive it is (up to the frequency limit if the amplifier it uses), so 100V at 60Hz produces the same output signal amplitiude as 10mV at 600Khz 

if the charger has a captive cable (doesn't use a detachable USB cable)
then it's probably not shielded and the signal may be from the powersupply itself.

Current clamps are for measuring how much electric current is flowing through a single conductor (eg to get an an indication that the circuit is not only live but is also in-use), as you need to separate out the individual conductors of the cable, you typically use them at a junction box or some other place where the cable is opened up.  using a current clamp on a bundled cable will usually give you a zero reading (unless there's some sort of electrical fault)