Teams try not to show anything they don't absolutely have to, whether they have secrets there or not. It's just how they roll.
Mercedes, however, do have something called a "magic paddle". So they do have an actual reason for keeping the back of the steering wheel away from prying eyes.
Here is a short video clip of a reporter attempting to get Mercedes GP's team principal Ross Brawn to disclose the details of the "Magic Paddle": http://www.youtube.com/watch?v=luWuJx2Ukik
There was speculation that the call was related to Schumacher's use of the DRS. We'll probably never know for sure.
Because since it's hidden they don't want to disclose that information to other teams. I recall DLR(McLaren's driver) being asked about it after explaining their steering wheel's controls. Anyway, IIRC the interviewer "guessed" that it was another way of activating the DRS, so in practice it's not really so secret, but F1 its like this...
Maybe there's a Ferrari sticker there.
The stickers that scare me the most are the Microsoft stickers on the ECU's...