We went a-wandering yesterday and no sign of anyone at CSC or HSC or the Crow's Nest, so it may be that Cowboy and Kate did leave us when the prey returned and that its one of the Radisson or West Winnipeg birds taking a break. We ID Jules at this location in the spring so we know she goes there. I'm inclined to say it was a Radisson bird, though we haven't been able to ID them by their bands here this year - Jules and Beau continue to be in their territory pretty consistently since the start of nesting.
Its a bit early for birds from away to be turning up, give it another month, month-and-a-half and then it could be a resident bird or a traveller.