By default, each time a user connects or disconnects from a database, the <dbname>.iqmsg log records this action.
You can control logging of user connections and disconnections using the database option, LOG_CONNECT. If connection logging is disabled when a user connects, and then turned ON before the user disconnects, the message log shows that user disconnecting but not connecting.
For more information, see Reference: Statements and Options > Database Options > Alphabetical List of Options > LOG_CONNECT Option.