The lack of some form of EPIRB (can't find confirmation or not of whether the vessel has on anywhere) would be criminal negligence in my mind. If the sub ends up being found intact on the surface days (or weeks) later it would be unbelievably tragic.
I think the most likely scenarios are probably total hull integrity failure during initial descent (60%), the vessel either becoming entrapped or the drop-weight system failing (30%), or having surfaced after some sort of systems failure and lost at sea (10%).