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.
The fob doesn’t turn off.
The car is always calling out for a response and the key “hears” the call and responds with their agreed upon codeword.
A faraday is like plugging the key’s ears and putting a gag in its mouth. It can’t hear or say anything.
… Which means that if the hacker is near you when you park - there is a time period where the fob isn’t masked by the bag, because it is coming out of the ignition, and voulaa - you can record the key’s pong of the car’s ping, retransmit, and get in. Correct?
Modern fobs should be designed to prevent replay attacks (there should be something specific in the request that alters the response), so it shouldn’t be possible to record a response and then use it later.
voilà
That is a possibility if you aren’t normally keeping it in the bag unless being needed in the moment.
It is far more common for the attempted theft to occur late at night because thieves avoid greater risk.