open secret
noun [ C usually singular ] uk/ˌəʊ.pən ˈsiː.krət/ us/ˌoʊ.pən ˈsiː.krət/
公开的秘密
a fact that should be a secret when in fact a lot of people know about it
It's an open secret around the office that they're having an affair. 在办公室，他们的绯闻是公开的秘密。