XP Pro Administrator account not in administrators group.

I ran into a very strange issue today on a workstation and wanted to run it past you guys. Somehow… the local Administrator account on a workstation was no longer part of the local administrators group. We didn’t notice that until after we removed it from the domain. I was not aware that it was even an option to remove it from the group? Anybody else run into this? It pretty much locked us from doing anything with the workstation.