The security is not that important.
I have a Chinese cabin scooter that only have an on/off button to start the scooter with, it does not care if there is a key or not, it is simply just a normal boring switch. To prevent people from just breaking the side window and drive away, do I want a power breaker someway, I was first thinking of a simple hidden switch, but what if I forget to switch it off?1 Then did I think of some sort of magnetic switch, but this again demands something magnetic being nearby I can use. I then thought of RFID, but what if I lose my keys. So my final thought, what about a simple phone detector, one that just triggers if one of the accepted phones is inside the scooter.
Nobody would begin to investigate why the scooter won't start, if they can not see a switch or fast find a solution, therefore this fairly insecure but hidden solution. :-)