Here in the place I work almost all of the people have experienced it. It’s very frequent in meetings, or in conversations in the hallways.
It consists in pretending there’s a floating computer keyboard in front of you, and you are typing on it, to illustrate a past moment when you had an IM chat with someone else. For example, when somebody says: “And I told him: Please come to this meeting”, while “typing” on this inexistent keyboard.
I think it looks weird, or at least funny, but it appears I am the only one who thinks that way, because nobody else notices.
Maybe I’m the weirdo for noticing that kind of stuff.