A ninja-turd occurs when your feces has the rare but precise trajectory to exit your anus and travel directly into the toilet pipe so that when the defecator turns to examine his/her excrement it remains hidden as if it was never there...(like a ninja)
Defecator: Wow I just pooped, let me see what I did.
(Stands to turn and look)
Where did it go? I know I pooped, but its just not here...ahhhh damn you ninja-turd!