Walls have ears
This is an admonishment to be careful because conversations can sometimes be overheard. The expression dates in this form from the early 1600s but the concept itself may be very much older. Some source maintain that it dates back to Dionysius, ruler of Syracuse from 405-367 BC, the very same ruler responsible for the sword of Damocles idiom. While most people know the story of the sword that Dionysius hung above Damocles’ head, relatively few have heard about Dionysius’ Ear, which according to Cicero, was a large ear-shaped, underground chamber that Dionysius had built beneath his palace so that he could listen in to conversations. The De Medici’s were thought to have something similar built into their Louvre Palace in Paris during the 1500s.