The last house I owned had a similar issue. There had been a house on a one-acre site. The owners split the site in two, built a house on the other site, and sold it. Alas, waste pipes (shower, dishwasher, rainwater guttering) ran out from their property, under my front lawn, through the hedge at my front, and into a roadside drain (aka dyke) via a visible pipe.
The engineer that I hired to survey the property discovered this and noted it. Said it could be a problem. As we had our hearts set on the property and the price was very appealing we turned a blind eye to the issue.
We sold that house two years ago and during a meeting with our solicitor, in an off-the-cuff comment I mentioned this issue.
My solicitor raised her hand and said, “I didn’t hear that. Now that I’ve heard that, I’ll have to reveal it to the new buyers. This could be a problem. Send me an email when you get home to remind me”. I forgot to send that email.
The new buyer dragged his feet for a long time on a number of matters and refused to have the house surveyed. Said he’d an eye for these things and the house was sound.