What does it mean: "like a ninja"

[harriet_yang]

It's from a movie called "500 days of Summer." Here is the quote:

"Tom: [split screen scene on the train about Millie's wedding] Yeah, but you said you were going that's why I'm going.
McKenzie: And that's why I called her last night, told her I was sick. Like a ninja. "

Tom and McKenzie are friends. They agreed to go to a friend's wedding but McKenzie changed his mind. What does "like a ninja" mean here?

Thanks!

 

[Skrej]

McKenzie thinks he's being sneaky, like a ninja.

 

[Tdol]

It is a very poor example of the ninja arts, but the person is trying to make themselves look smarter than they are.