A Ninja is someone who leaves a party or pub/club without telling their friends, all stealth-like (usually because they're tired or drunk and don't want to cop flak for leaving early). This practice can become a common prank between friends, sometimes blowing up into an all-out "Ninja War".
Friend 1: Where'd Beamers go?
Friend 2: Umm, she told me she was going to the bar for another drink. Wait... That was half an hour ago...
Friend 1: Unbelievable, she's done a Ninja again.
Friend 2: That's it, Ninja War declared.