UserID blank with AD integration

I use Active Directory integration, IIS auto login and dynamic permissions. I have a link that sends users to an add page:

http://mydomain.com/trainings/StaffTrainingSurvey/add?t=635

but it appears that XSession.Session["UserID"] is blank when I send them directly to this add page. If they go to the root of the app (i.e. http://mydomain.com/trainings) and then click the link to go to the add page, this session is populated.

Also, all authenticated users have access to this add page, but guests and default is no access, though they are able to load and save the add page even though the UserID session is blank.

Shouldn't the XSession.Session["UserID"] always be populated? And when going directly to the add page, doesn't it auto login in the background and then bring them to the original link?

Thanks,
Tim