It is obvious in retrospect because this concept has been around for 30 years. A common killswitch mechanism that I remember being implemented in the early 90's was a system that tied into accessory devices. On my friends car you had to put the key to ACC, then turn the cruise control on and off, and then engage and disengage the parking brake before the car would start. No other obvious lights, buttons, switches, etc. And you could install the killswitch device to tie into basically any 2 systems that used battery power.