Perhaps I am a Luddite but I wish the Ioniq were not keyless ;-) There's something satisfying and purposeful about putting a key in and turning it (or turning the "engine" off and pulling the key out).
I too have forgotten a few times but the beep was loud enough in my case to remind me. The audio not going off is also a sign. I kinda wish it would shut off when I shut off the vehicle instead of waiting till I open the door.
I've noticed that the vehicle can be driven without the key fob (it will whine) if it were turned on with the fob initially; I've not tested this but wonder how long it might drive? Seems like a big security hole. In my hypothetical nightmare, I leave the vehicle on while I walk away and someone drives it forever by leaving the vehicle on and fueling/charging it as necessary. ;-)
I don't like to feel anything in my pockets while driving. The keys go into the cup holder, also does my wallet and my phone goes on charging.
That said, it already happened to me to exit the car and push the door button to lock the vehicle.
Fortunately, they handled the stupid moves that I unconsciously like to do, by beeping so hard that you can't even miss it.
I guess it is the same beep when you are trying to lock it while it is still turned on.
By the way I agree with you.
Personnaly, making everything keyless makes me forget to lock the vehicle (because keys in pocket) and 1 time over 5 I receive a bluelink notification "the vehicle was not locked". The app is only good at this thing though.