Active Directory status is a standard inventory report item in the Casper Suite. You don't need an Extension Attribute unless you were looking to pull something much more specific than just the domain the Mac is joined to.
The AD status does not seem reliable to me. I am seeing 20 machines which I know are joined that are not showing in JAMF.
Are you certain they've recon'ed to your JSS properly? Are your Macs joined to AD at imaging time or are they joined later? There's usually some reason why data isn't showing up in the JSS. I'm just not sure what it would be on those particular Macs
We are joining manually post imaging.
Kind of reiterating Mike's question: Are you running recon after joining your Macs to AD so that the JSS gets updated to reflect this information?
If you have something like Apple Remote Desktop you can run a UNIX command to display whether a machine is bound to AD:
dsconfigad -show
To get just the relevant information:
dsconfigad -show -lu localAdminName -lp localAdminPassword | head -2