"Many auto thefts are crimes of 'opportunity.' Leaving the keys in your car (even a 'hidden' spare key) greatly increases your chances of having your car stolen. Leaving your vehicle unoccupied with the engine running, even for a minute, is illegal in itself; but it greatly increases the chance that your car will be stolen. It is truly a helpless feeling to watch your car being driven away by a total stranger as you stand 10 feet away at an ATM or mailbox. Take your keys with you even for quick trips. Needless to say, this is especially true if you have children or pets in the vehicle."
Courtesy of the Seattle Police Department. Suggested by my brother George.