As per Davesparks the voltage you read could be anywhere between zero and 230 depending on the potentials of the connected cables that the floating cable is lying next to. The actual voltage is pretty meaningless because it's not part of a recognisable circuit. Testers with different impedances will give you different numbers, typically it will be somewhere between zero and 115.
The current that you can draw from it will be microamps which is why even a high voltage is unimportant. You can charge a balloon up to thousands of volts by rubbing it against your jumper, but the charge stored is so low you won't get a useful measurement with any normal instrument. Like the balloon, the floating cable is just a conductor with some stray capacitance to things nearby.