OMG I excprieneed this same thing while visiting friends in Germany. The house got too quiet and sure enough the front door (same locking situation as yours) was wide open and both of our 2 year olds were gone. Thankfully we had caught them early and they were only playing in front of the garage of the next house over...but it was the most awful feeling ever. From then on we'd lock an interior door AND put up a baby gate!