Sorry Raoul
I really don't get the connection or what you think the sign you refer to might mean.
In the seventies when there was a rash of hijackings, some comedians did a cartoon or skit about a guy waving at a friend in an airport and shouting "HI JACK". It caught on and some funny guys thought it was witty to act that out at airports. Hence the signs and warnings that joking about hijacking is not acceptable, being the equivalent of shouting fire in a crowded building.
What that has to do with the French sign is a mystery.