This is likely a long shot but, without knowing your lot/house set up in relation to that drain and everything in close proximity, it might be worth looking. Have you confirmed that you don't have a water leak in your house system that is somehow making it to that drain pipe? Sometime when you know there is no water running, filling a hot water heater, water softener, etc..., lift your meter cover and see if the wheel is spinning. Not trying to insult your intelligence with those instructions....just offering something that might be worth ruling out and is fairly easy to accomplish.