Round trip times? Can't cheat the speed of light, assuming you can't spoof the transmission.

The car could log the signal strength of all successful auth attempts from the key fob and determine an acceptable range. Anything outside of the acceptable range could then require use of the mechanical key (for those fobs where the mechanical key is built in) as a precaution.

Fobs could require a 'wake up' key press after a certain duration of inactivity.

Fobs could have a physical switch on them, enabling an airplane mode.

These ideas all give up some manner of ease-of-use.