I'd hazard a guess the property would be too modern to have combined drainage, so there should be a seperate surface water drain somewhere. Where it runs underground from the downpipe is impossible to tell from the photo, it could still run to the rear.
Could you post a photo from further away showing the butt, door, downpipe etc in one pic so we can see where they are in relation to each other?
Could you post a photo from further away showing the butt, door, downpipe etc in one pic so we can see where they are in relation to each other?