Not entirely relevant but note that if a power switch does not have a mechanical difference between the off and on state then there must be an active component that at the very least must monitor the power button to detect presses. I don't really know, but it seems likely that in a smart phone the SOC is used to monitor the power button and it is therefore only ever in a very low power state (sans battery removal). That doesn't of course mean that any radio components are receiving power.