As someone may have pointed out, the owner(s) aren't boaters, and the broker has been running with skeleton staff over the break. So neither would necessarily have time or the knowledge to think about walking the towpath, although my first thought would certainly be to cycle the towpath, after guessing which way it had gone. Even at my level of unfitness, I can do double the speed of a boat on the Grand Union.
The nearest common equivalent event for most people would be their car going missing from where it was parked, in which case, the assumption would be that it had been stolen. You'd possibly not think to check a couple of hundred yards down the road to see if it was there. Just don't ask.... I still get hammered by my car insurers 5 years later for just reporting the theft, despite there being no formal claim or payout being made. (Security tip:- Buy a car with a choke. Most thieving scrotes don't know how to use one. )
Unless the broker has control over the place where the boat was moored, then they have no legal responsibility to keep track of it 24/7, and the way that the broker reported the boat being found within a day or so of it being reported missing is a point on his favour, if anything. I'd certainly not count the episode against him in this case.