When you're on Live Messenger and you tell people you're signing off then you change your status to "Appear Offline"...like so people think you have signed off but you're actually lurking in the msn shadows, waiting to pounce when someone cooler signs in.
Rach: "Here how can you be talkin it says ur offline"
Gill: "It's cool im on ninja mode so nobody else annoys me."
When a person, or group of people for that matter, enter a social establishment or gathering, with the intent of getting inebriated without the social and relationshipial backlash of having ones photo's tagged on Facebook.
Joe "Dude lets hit up that crazy rage out in Hutchinson"
Jack "No way dude, my GF will creep on my wall and see me tagged with a bunch of random hotties"
Joe "Dont worry about it, we'll go Ninja Mode"