I'm not sure I follow. You just want to add a history entry that says the late activity ran? The system should do that for you by default. If you want something more custom, your activity could use the History-Add method to add your audit log. None of that should require calling a flow. If you are trying to resume the flow that the work object was on, calling the activity ResumeFlow would be the way to do it. If you want to call a brand new flow the Flow-New method is probably a place to start. If you can give us a bit more information about what you are ultimately trying to do, it might be easier to give you advice with confidence.