In the document outlining FIFA's experiment with Video Assistant Referees (VARs), a referee signal is defined to indicate a review is in progress by "showing the outline of a TV screen" (I guess that means with his hands or fingers and not by pulling a picture of a TV screen from his card pocket). But -- "A player who shows the TV sign will be cautioned (YC)". Considered to be dissent, I guess, or maybe a new method of showing the ref the finger.
http://static-3eb8.kxcdn.com/documents/216/VAR_Protocol Summary_v1.0.pdf
http://static-3eb8.kxcdn.com/documents/216/VAR_Protocol Summary_v1.0.pdf