I've got a 240V water flow switch connected to the hot water pipe driving the shower - it has 3 cables (COM, NO, NC). When I connect a permanent live to COM, both NO and NC are live. It doesn't make any difference whether the shower is on or not. I don't understand how is this possible.. is it most likely that it's a fault switch?
Most likely, your non-contact voltage tester is fooling you. Such devices are not perfect and everything inside the switch is probably so close together that the tester cannot really tell.
If you have a voltmeter, try testing with one lead connected to neutral and the other on NC and then on NO. (Note that with a high-impedance input volt meter such as most digital meters, you may het a phantom reading on the open contact though it should be much lover than mains voltage.)
You can also try temporarily wiring in a light socket and test with that.