Around the kids, when they were young, we used French for those things we didn't want them to know about. She's fluent and I don't really speak it at all, but took it back in High School and remembered enough vocabulary to make it work.
Today, both our boys have taken years of Spanish and they speak in Spanish for those things that they don't want us to know. LOL
Around other adults, my wife and I have a system of looks that we give each other. We can have an entire conversation just by giving each other looks.