With SSOMode enabled, users are able to use the sspsession cookie to access resources without being re-challenged.
There are no Indication within the Existing authentication audit events that indicated that the user was allowed access based on SSO.
Any release prior to 3.4
Starting with 3.4 release, if the user is authenticated by SSO, an authentication event (with factor = SSO) and/or SSO event will be issued to capture the fact that user did get authenticated by SSO.