I have a sewer-cleanout near the back porch of my house, which is below ground level so it's in a round box, and also within this box I have found a smaller piece of pvc pipe. The square 'handle' part of the sewer-cleanout has been cut to allow this smaller pipe to point into the larger drain.
To try to figure out where this drain comes from, I have run every faucet and drain the house and also used the sprinklers and I still haven't seen water come from the small drain. (I also verified that this is not the Air Conditioner drain line, that goes another place.)
Any suggestions on finding where this drain comes from, so I can be certain that it's safe to cap it?