IIRC if a TG is commanded when you enter the arena their CS may not show. Not sure if you can take it from them with a lower ranking when this happens.<<<<anyone else know?
You are right. If someone's in command of a TG when you enter arena, his name will not show. But you can still take over command if your rank is low enough.
The only exception to that is the first day of a new tour, where many players still have rank 0 - in this case, you might not be able to take away the command from them.
And of course, often players do set TG course without bothering to actually "take command", so you can see the TG changing course without any name in the command list, too.