Information
For completeness of forensic analysis, it is necessary to know how long a user's (or other principal's) connection to SQL Server lasts. This can be achieved by recording disconnections, in addition to logons/connections, in the audit logs.
Disconnection may be initiated by the user or forced by the system (as in a timeout) or result from a system or network failure. To the greatest extent possible, all disconnections must be logged.
Solution
Add the 'LOGOUT_GROUP' to the server audit specification
USE [master];
GO
ALTER SERVER AUDIT SPECIFICATION STIG_AUDIT_SERVER_SPECIFICATION WITH (STATE = OFF);
GO
ALTER SERVER AUDIT SPECIFICATION STIG_AUDIT_SERVER_SPECIFICATION ADD (LOGOUT_GROUP);
GO
ALTER SERVER AUDIT SPECIFICATION STIG_AUDIT_SERVER_SPECIFICATION WITH (STATE = ON);
GO