I've had to deal with this recently. For 20 years I was driving a 1998 Civic with a simple old-style key. It went in my BCD pocket when I was diving, never had an issue.
A couple years ago I bought a 2018 Civic. It's very fancy. Push button start and no key for anything... just the electronic fob that the car senses is nearby. It can't go in the water, and I can't even leave it in the car: the car knows that if the fob is in the interior and will not allow the doors to lock if it is.
I tried hiding it under the back bumper, but when I do it's close enough to the trunk to allow anyone to open it. Solution?
I hide it in the grille of the front bumper. It blends in well (everything's black) and it's just far enough from the doors that they won't unlock automatically.
I'm going to try playing around with other options, including using a small metal box (to diminish the radio signal from the fob) to hide it in somewhere under the car.