The first suspicion is that it might be a paid for DLC coming, but apparently the mode is already accessible via a “simple” button combination “entered on one of the game’s menu screens.”
Senior multiplayer programmer Peter Knepley (Epic Games) said via Twitter that the mode’s inclusion was simply a mistake – leftover game code after being ported over from Gears of War 3. He went on to say that Warzone is “not the polished Execution of April 2nd.” Whether the mode will be officially unlocked, he continued: “I’d never say never, but Execution is way more popular than Warzone.”
On top of this, there appears to be references to additional maps too. An image posted on NeoGAF shows references to Mausoleum (possibly a re-make of the Gears 1 map), Hospital, Museum, Capitol and Haven. The last two are already confirmed for the Maxim sponsored free DLC arriving on 2 April alongside the returning ‘Execution’ mode.
Players have been able to access and play it using a simple button combination entered on one of the game’s menu screens.
Warzone can be accessed using the following method (and there are numerous video walkthroughs on YouTube):
After booting the game, choose Private Match and highlight Team Death Match, and press A. At the next prompt press the A and B buttons simultaneously. When it says “You are about to start a match. Continue?”, press A again.