The boys have a cat-flap in the back door, leading into gardens. I don't let them out the front, because it's a busy-ish road. We seemed to come to an understanding that the front door is out of bounds, and usually they eye the front door with suspicion when opened.
Tonight, I opened the door to the Tesco delivery man. Little Monsieur sauntered out. and ran across the road in front of a car!
I've learnt my lesson, and will be shutting them in the kitchen when the front door is opened. I was running around bare-foot in my onesie, trying to catch him like a mad thing, then sobbed for about an hour afterwards.
Lesson learnt.