At the place where I used to work, people in high positions would walk in there jabbering away to other people in high positions, get their coffee and their lunch pails, then walk right back out, still jabbering away. Me and everybody else heard plenty of stuff that we should not have been a party too. Later in the day, you were guaranteed to hear someone else, in another part of the building say, “You won’t believe what Mr. Jensen said in the break room.”
But to respond to your question a little bit better. I have found that when 2 people are sitting next to each other (on the couch or in chairs or even standing by the coffee machine), quite closely and looking directly at each other, they are “attempting” to have a private conversation (even though they might be talking loudly enough for everyone to hear) that you are not supposed to be involved in. It’s silly, but that is what I have observed.
If these same 2 people are sitting across the table or across the room and they are not right next to each other and may not even be looking at each other directly, then it’s pretty much assumed that the conversation is open to all takers.
But with the first example, I have often seen it done (and done it myself) by saying something like, “I couldn’t help but over hear what you were saying about such and such and I think X,Y and Z” But you must throw in the part about couldn’t help but overhearing. If you just jump in without saying that (even though they were clearly talking loud enough for everyone in the room to hear) they will be offended and give you a dirty look or comment. This has just been from my own observations.