Essentially what the sibling says. Does your expectation of privacy change when: you're taking the subway? When you're in an empty car? When you're at a concert packed with people? When you're tucked away trying to escape the crowd? When you're in a public restroom? What about if you're walking alone in the middle of the night and no one is around? What about in your house with your blinds open? Or in your front yard? What about in your front yard if you live 5 miles away from your neighbor?

If your expectation of privacy changes in any way with any of these options, then yes, you have an expectation of privacy in public. It is a spectrum, not a binary decision. But if you think it is a binary decision you're welcome to try to record people going to the bathroom in public restrooms and see what happens. I will not be covering your lawyer fees.