i am not 100% sure but here an advice if your device operates near people and might harm them. (note that arduino shouldnt be used in such environment)
however in such environments and as an addded saferty like such environments.
Besides electrical discharge, it it is for safety reasons then your sensor output to keep a machine going should always be on (provide signal).
In other words if you loose signal the device should stop whatever it is doing.
So even an "on" button cannt be a single push signal, and there cannt be a on signal to turn something off in a flow of behaviours.
no signal is then interprented as broken signal, and thus a reason to stop or to behave different.