You'd probably have seen a crack if you'd inspected the frame before the failure, but I can't imagine most people inspect their motorbikes that closely. Even if you did, the layer of dirt that builds up on stuff might hide the crack, or you might mistake it for a scratch in the paint, plus if it's at a weld, it might not be particularly easy to spot anyway. The point is, there would have been a warning, but it's not a warning you'd have seen.

And there lies the rub, so to speak. Carbon can break suddenly while feeling normal and looking fairly decent, while metal would creak and/or crack visibly, if it didn't bend.