general problems of the 2001 Subaru Outback

While parked in neutral, with the emergency brake on, and no occupants, our standard transmission vehicle rolled backwards out of our driveway, across the street & into the yard across the street. Our driveway has only a very slight grade (less than 5%), so I'm alrarmed with the ease with which this happened. There were about 8 children playing around the street & yards. My neighbor & I both confirmed that the emergency brake was in the upright position, which I always engage before exiting the car. We do not know if there is a problem with the emergency brake itself, or if I simply did not pull up enough on the brake; however, if this can happen to a safety hound like me, I'm assuming I'm not the only one & that it's a common enough occurence to warrant some kind of indicator to alert the driver as to whether the parking brake is actually engaged.