I'm not sure if there is already something like this, but it's exactly what it sounds like.
You have your admins set in the admin controls menu to who becomes admin automatically when they join the server like usual, but there is an extra option that allows you to set an auto-admin to "undercover". This means: when they join the server it doesn't show them as becoming admin in the chat, in the player list there is no A or SA next to their name, but that player is still an admin/super admin. Then can still use the admin controls window, ban or kick players, etc. They can do everything a normal admin can do, but they are undercover.
This could be useful if you want to try to catch players who are breaking rules because they see there is nobody there to punish them (yes, a lot like what happens at Tezuni's or Cat's server). This way you can catch rule-breakers in the act without them knowing an admin is in the server. It's kind of like sending an undercover cop to buy some drugs, but then they bust the guy who is selling. They wouldn't commit the crime if they knew there was an on-duty cop trying to buy it, that's why he's undercover.
I was also thinking that bans and kicks would be anonymous in the chat (not in the unban list) so nobody knows who the admin is after they ban or kick someone. It would say "Undercover (Super)Admin has banned/kicked Blok" or something along those lines. It would also not show who banned you in the ban message window. The admin would be completely anonymous (except to other admins).
I'm sure you guys could find several negative things about having this, but I still thought it would be pretty clever to have.