What is the word for something that only moves when you aren't looking at it?

You know, like a Weeping Angel from Doctor Who or the mannequins from Solitude in Skyrim?
Asked Feb 16, 2015
Edited Feb 28, 2015
A mannequin... definitely a mannequin.
Answered Feb 25, 2015
That's exactly what I'm talking about lol! But what is a thing like that scientifically called?

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