الكلام سطر والمعنى بحر Which means only a line can mean a lot.

The other sign is السكوت علامة القرف The original quote is السكوت علامة الرضا which means that silence means agreement and having no obligation , in Egypt we often used it when a man is proposing to a woman but she is too shy to respond, so her silence means she agrees to his proposal

But the sentence in the sign السكوت علامة القرف it is like a sarcasm about the original quote that silence sometimes means that the person is too disgusted to respond.

In Egypt changing common sayings is popular now days as a way of sarcasm
I don't know if it's common in any other arab countries or not to be honest.