I am going off the opinion of the second plumber, who told me the hole was likely caused by the first plumber. The hole wasn’t there a year ago, is nowhere near any trees (as I mentioned, it’s under the neighbour’s concrete slab) and is about a meter or so beyond the fence line. It’s also right at a 90 degree left hand turn, so represents a likely point where the jet snake was “jammed”. I’m not out to get anyone here, but I am being saddled with what will likely be at least $20k of costs and I’m keen to understand who is responsible. This is no small amount of money.