In a house I am about to renovate, there is a strange plastic pipe in the outside toilet. My house has flat-roofed kitchen extension including the loo, and next door has an ajoined pitched roof extension. There is a black plastic drainpipe running the width of my outside loo, terminating in a grille on the outside wall next to the door. The other end of the pipe seems to go into the wall, as if it is going through to next door, but I cannot see if it goes in or not without digging out some brick etc. Eventually I will be able in investigate properly, but, has anyone seen anything like this, or know what it could be ?
ASCII art follows, please view in fixed font.
___________________ | _____ | ventilation | | | | grille -> :===========|