When a user is timed out from inactivity, while it does log the user out of the system, it isn't recorded in the same way as if a user were to log out of the system with the logout button. It closes the connection between the client and the server, but it uses a different logout code.
The code used will be 'EXPIRED'. This is used in the table DT_USER_SESSION_HISTORY, where there is a column 'Logout_Status_Code'.
If a user has a timeout, they will get the 'EXPIRED' code logged next to them in this column.