Voltage tester detecting voltage in non-hot wires when the light switch is off
Yes, it is AC voltage that is transfered by the tiny capacitance between galvanically isolated wires. The test equipment (normally having an input resistance of a few MegaOhm)  is very sensitive and can measure these voltages, because it does need only tiny currents. Physically, the voltage source has a huge resistance. 

Another example would be the natural E field all over this planet, which is equivalent to ca. 100V per Meter height difference. There would be ca. 200V between head and feet of every human being. But this voltage source has an even higher resistance, and needs a very sensitive special test set.