[NicholasJohnson]

I think it depends on the type of car. Modern day fobs are in a "give and take" type of action for lack of a better term. The car is constantly asking where the key is using a certain signal, while the fob waits for that signal to reply. Its what causes some of the cars at night to light up when you approach. The only way to prevent it is to park it in a garage sad to say as key cloning has been simplified with the advancement and availability of scanners and readers.

 

[NicholasJohnson]

A Faraday pouch can't hurt - and it will also block the fob from communicating with the car until you want it to. The fob is the transmitter; the car is the receiver (thus the button battery in the fob).

I use them - the foil lined Tyvek ones - for all my various "credit" and gasoline cards and we have proven that they prevent unwanted scanning.

faraday bags for credit cards - Google Search

Doritos Cool Ranch does the trick.