Its entirely possible that one of the boys (Rosser) in this case would have taken off early ... it happens frequently that one bird is just gone - Hurricane is a case in point, we saw her only twice after fledging. Obviously she was still around and reliant on her parents until she could hunt on her own but she had the wanderlust first and it did her no harm whatsoever. Radisson and Alley I think both had the same urge. I've been watching the hawk migration stats and there are alot of peregrines on the move, so it is entirely likely that the boys' feet are itching to go.
As for Hurricane and Brooklyn, they are the resident pair in Brandon and more experienced so they will leave when they are good and ready. Last year at the Radisson, Sandy from Fargo turned up and then she was back this spring with an eye to taking over the Radisson nestbox - hanging out and defending your territory in the Fall could well mean less conflict the following spring from younger interlopers.