Halo 2 for Vista was found to have a hidden part of the code containing a character which moons the player. Much like GTA's hidden Hot Coffee code, this "Easter egg" is a piece of the code, likely a joke from one programmer to another, is only accessible by someone who is very familiar with programming and has a lot of time on their hands. This should not be confused with unlockable levels or characters, which are common to many games and a completely kosher part of game design.
That said, after the fallout from the Hot Coffee controversy, the ESRB did change the terms of the contracts that it signs with software makers to mandate disclosure of this kind of inaccessible code. But was this change truly motivated by market forces?
The ESRB certainly isn't operating in a political vacuum, but is rather well aware of its political circumstances. The ESRB, like the MPAA, knows that its voluntary rating structure, to some degree or another, operates at the pleasure of Congress. A few more Hot Coffee scenarios may result in Congress stepping in and mandating certain standards, like those proposed by Senator Brownback, which would cripple the ESRB system.
Anticipating possible moves of Congress is part of the ESRB's policy and this can only harm the ratings systems efficacy. Good ratings are about conveying information, not about regulating content. If the ESRB continues to be encroached upon by legislation or the threat of legislation it will start to become crippled like the MPAA, which is now considering the presence of "glamorized smoking" in its ratings process and seems to be on the road back to the Hays Code, which inspired the formation of the MPAA's current rating system three decades ago.
While I understand that the ESRB may make the same moves in the absence of an abusive and unconstrained Congress, it's unfortunate that we're living in a world where it's impossible to know.