What is the meaning of walls

the walls have ears

Examples:

»While the Cubans stood in line for food, we employees and the diplomats [...] lived like maggot in bacon. Free visits to the cinema and bar, cocktails and banquets replaced each other, but the walls had ears, even in the churches, "reports Hans-Udo K [...], referring to the wiretapping practices of the Cuban secret service. [Fränkischer Tag, 17.09. 2011]

She herself, says the woman, who grew up in Istanbul but has lived in Germany for 26 years, lost her grandparents to the genocide. It was never discussed in the family. Her [sic!] Father always said: "The walls have ears." [Südkurier, April 27, 2015]

We did not learn what revolution means in Arabic. After all, there was still talk of uprisings from abroad. A little turmoil that would soon be over - as we were told. However, we also learned "the walls have ears". Because in Syria nobody knows who is listening. [Die Zeit, 06.05.2011, No. 19]

Hertha Borchert was more than just the mother of a famous son [headline] Today the narrator and "public enemy" would have been 100 years old [headline] March 1933, in a stairwell at Tarpenbekstrasse 82: A woman expresses her displeasure with the bold walk through the city marching Nazis with the brown shirts. But the walls have ears - a roommate hears your comment, which will probably come up more often later on. Only two and a half years later does the man make use of it: He denounces his neighbor [...] in order to gain advantages.