Domain Users not showing up in "memberof"

When I do an LDAP search for my user, why doesn't the "Domain Users" group show up in my users' (or any of our users') "memberof" attribute?  I can see the rest of the "plain old AD Security groups" that I'm a member of...but not Domain Users.
 
 
 
When I do an LDAP search for the Domain Users group, it shows up....but it doesn't show any "member" attribute....so does that mean that the Domain Users group is somehow treated differently than a "normal" AD security group?  Why doesn't the Domain Users group show up in my user's memberof?
 
 
 
If this is the wrong forum, please tell me which one I should post to.
 
 
 
Thanks in advance for any help you can provide!

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Rich_12