Here’s how I understand the issue:
A keyfob is a radio Transmitter. To unlock your car you need the radio transmission to reach the car. The keyfob doesn’t transmit a signal when at rest. Therefore putting a keyfob in a Faraday bag achieves nothing.
I have proximity unlock and my car is parked (at home) just close enough to my catch-all tray where I store my keys to constantly lock and unlock. It usually teeters on lock/unlock so closely that a person walking between the two will trigger it. So, to prevent a dead battery or stolen car, I keep my fob in a faraday bag.