Is it possible that someone has made changes in model but their username is not visible in model history.
Activity records are present in history but username column is blank for some records?
User name will always show up. As far as I've seen, only when changes are made to the user list itself (settings -> Users) the user who made the change does not show up in the log
The user name does not appear and entire line in the history is blank except description "Model change (no details available" when there is performed an action of edit, adds, deletes or rename of an object outside of the following list: Action, Currency qualifiers, Dashboards, Functional Areas, Import Data Sources, Import source mappings, Line item subsets, Line items, List properties, Lists, including list items, Modules, Processes, Revision tags, Roles, Saved views, Subsets of lists, Time settings, Undo operations, Users added or deleted, Versions, according to Anapedia.
An example I could give is when the model is created for the first time and it is shown in the history the users added to the model as a part of its creation.