In general, when attorney and client have a conversation in the presence of a third person, that third person can lawfully testify to whatever was said. But the same isn’t necessarily true when the attorney and client couldn’t have reasonably known of the third person’s presence.
Example: Russell and his lawyer, Maurice, meet at a coffee shop to discuss the former’s indictment. Officer Daniels, whom they don’t recognize, happens to be seated at the table immediately next to them. He can easily hear their conversation. At the eventual trial, Daniels can probably testify to Russell’s statements during the coffee meeting.
Example: Officer Daniels sneaks a hidden microphone into Russell’s car. One day, Russell and Maurice sit in the car and have what they reasonably believe to be a private conversation about the pending indictment. But they don’t know that Daniels is listening to and recording the conversation via the microphone while hiding in a nearby van. In all likelihood, Daniels will be prohibited from repeating in court the contents of the conversation.