I agree - stash only a key blade that lets you into the Rover door.
A full key fob stashed/hidden in an interior cubby could then start the vehicle.
As far as I understand, a battery is not needed to start the Rover with the fob key - the "handshake" is passive.
The battery only powers the fob lock/unlock/buttons.
I suppose you could rotate your fobs to keep the stashed one topped up if you want, but should not be necessary.