Audit Database > Logon Report > Empty Session(s)?

[PolzovatelCognos]

We utilize the Cognos Audit database to keep track of user activity, etc. We created an Administration dashboard to assist us with Cognos Administration. The dashboard contains several helpful daily reports: Today's Logon History, Today's Report Errors, Today's Report Execution History, etc. On the "Today's Logon History" report that we created off of the Audit database, we have noticed that occasionally, the report will display a Logon time with a blank user session. Any ideas as to what may cause for this "blank session" record? We utilize the LDAP server for user authentication for Cognos and only users in our domain can access the application.  Thanks.