I'm a police officer who just retired from a VERY expensive high end area in NJ. Our area had so many high end vehicles (MB AMGs, BMW Ms, Porsches, Ferraris, etc. ) stolen right from driveways or garages because the key fob was left in the vehicle. I do not recommend EVER leaving the key fob in the vehicle. The conclusion that my county came up with, and I agree with it... Is that there is a tool / scanner that criminals now have that can pick up the signal between your car and it's key fob when the key fob is within range of the car. You have to understand that when a proximity key fob (no key required to enter or start a car) is in a car, it's sends a signal to the car telling it that its ok to start and operate the car. This signal is transmitted and then intercepted by criminals who will be informed that a car in the immediate area has the key with it. They then pinpoint the signal and are able to get into the car and steal it. Just in my area in the past 3 years,at least 30 cars were stolen when the key fob was left in the car. While maybe 4 cars were stolen in the same area without a key fob- type car or a key fob left in the car. IMHO, do not leave the key fob in the car. Of course, if you are just going to the gym or a quick errand, then maybe okay but do not make a habit of it. We had one family who lost 3 cars in one night worth over $250,000. And all had the key fobs left in the car.