I am afraid you just may be correct with the converter hub (how it got that way I wouldn't know).
For some reason, they just crack there. I think it has to do with the square cut corners being a bit of a stress riser, and giving cracks a place to start. If it is not a bad crack, it may be a little hard to see. Silver Seal made a spray crack detection kit that I had used, and it worked quite well for this.