Actually, on mine it appears to be a matter of physics, when the headphones are on a head, the earcups rotate to fit over ears, and the headphones turn on. When you take them off, a spring or some other mechanism rotates the cups 90 degrees so that they lay flat. This also turns off the power.
So rotating the cup is the on/off switch. I think if someone were wearing them and no signal was being transmitted, they would probably still be on, but it's doubtful anyone would want to wear them without listening to anything (OK, I know, kids are unpredictable . . .)
If they do shut off in the absence of a signal, I guess I've just never left them on long enough to find out. But I do know that they'll turn on even when the car is off (and thus no IR signal), just by turning the ear cup as mentioned above.
--Jason