On this point...
I've just been thinking that I have never ever taken my keys/phone into a cave. Phone is always tucked away in car somewhere and the car key hidden somewhere between car and cave, or just inside the cave entrance, or next the car in a wall or some such. Everyone on the trip knows where each others car keys are in case of emergency.
Granted, most of my caving has historically been in evenings mid week less likely to be 'spotted' hiding your key. Even then, most caving spots are fairly remote and if your subtle about it, shouldn't be a problem.
Certainly one less darren drum to cart about the cave.