Yesterday's unannounced GTA Online update which added a new car, free shirt and new discounts to the game has finally been unveiled by Rockstar on their Newswire. The update also includes a new Adversary Mode which wasn't made available before the official reveal went up.
The update was noticed yesterday after Steam users saw the client update the game with a 55.2 MB patch. The small size first made players assume it was just a handful of tweaks, however some eagle-eyed players noticed that a new car has snuck into the game. A few discounts were also noticed, alongside a free T-shirt awarded to anyone who logged on.
The whole thing smelled like a weekly event with a tiny DLC attached, however no official announcement was posted about it. While updates going live before the Newswire post isn't new, this was the longest time in between the two. The update was also updated (so meta) today with further content.

GTA Online has received yet another Adversary Mode, which is something of an amalgamation of existing game mechanics. Power Play is your typical team deathmatch spiced up with all kinds of crazy power-ups that hearken back to the heyday of competitive shooters. Big circular icons floating a few inches above ground will grant you various wacky advantages over the enemy.
Beasted will turn the user into the beast from Freemode Events, including all of the special powers; Zoned will activate bullet-time for all players; Doped will obscure the sight of enemies with a green haze; Raging will increase damage dealt and decrease damage taken for your whole team; Flipped will change the camera controls of the enemy team and scramble team chat and finally Dark will take your team off the radar.