Ninjas are appearing everywhere in Club Penguin. If you haven’t read the newspaper this week yet (issue 160) then look for a secret message inside. Click on the N-I-N-J-A letters in the headline about the lightning storm strike to get a special secret ninja message.
You can also see ninja shadows appear from time to time in many places on Club Penguin. Look in the lighthouse, the PSA HQ, the ski hill. They appear every few minutes. Also go to the town and click on the big N in the nightclub sign and a ninja will jump out. You can see all the screenshots and stuff on this Club Penguin Ninjas page.