Workitems can be assigned directly to positions or persons and I think that even to org units. if there is a user assigned to them good, if not it will still work. A user or person can be assigned later and the workitem will be seen by him/her - although you might need to execute agent rules again.
When you click on the agent in the workflow log, if everything is o.k. it looks the same - you see the user, ever noticed the 'complete view' button at the bottom of the agents display? if it's an assignment by position you will see the position->person->user links. if something is missing you will not see an agent although the workflow will not be in error, the 'complete view' button will display the links up to the missing one.