Modern keyfobs keep listening and transmitting all the time, as you no longer need to push a button. Just get close enough to the car and it opens.

A terrible "feature", since it means someone can steal your car just by relaying the signal from outside your home at night, or an accomplice walking near you as you're entering the grocery store, etc.

I've become a big believer in leveraging some security features of the physical world, as it seems it's been long enough that everyone's forgetting Therac-25-style problems. (Or, perhaps more accurately, nobody cares because they aren't liable.)