[Dezzy]

You're right about the corner MJ but I can see why they have done it, it's flanged down that edge to stiffen it, then when it was welded by the robot it took a single pass across the top. Really to make that joint better it should have had a relief cut (mickey mouse ears) into the corner, or welded around the return (corner) if the return had been caught there would have not been a path for the crack to develop.
That said it is quite a big weld and the gap there at the start of the pass would have been quite large for a single run, I bet the flaw was always there from the factory as a hairline crack probably not visible to the eye. Once there is a path