And certainly more minor property line disputes happen all the time--including situations where the infringement probably seems inconsequential relative to the heat it sometimes causes.
From what I could see, the builders most likely kicked the boundary peg out of the ground and just put it back willy nilly.
As a consequence, I did advise my son to have nothing to do with the property and we did inform the real estate agent of the problem. This would put legal responsibility on the estate agent if it was sold without the boundary line being corrected.
Due diligence is necessary if you are doing this kind of thing.