I tried in the training/local game mode, and thought I saw something happen at 30 kills. I noticed one of those challenge badges at the top of the screen that said "nuclear" underneath it, and did notice a few kills that happened against the other team that I did not cause with any of my weapons. I did the same thing a few times, and could not see any type of incoming munitions or any change to the sky (like the MOAB/Nuke does) when I reached that "nuclear" 30-killstreak point.
I can't find any videos online showing the "secret kill streak" yet. Have you guys seen anything?