If these are client assigned assets, then to see past assigned assets you have to search in the asset history
Under Assets - Advanced Search select in the conditions :History - Action - Contains - "Client assigned: David"
You will have to then exclude any current assigned, either manually or adding addition conditions to the query above. e.g. client username does not contain David.
I can't thank you enough for your answer. This was the last big thing I was trying to figure out before we switch all of our inventory into this system. So, thank you!!!