I would say that this is something you learn over time but I am not sure...

Even though I am a supervisor and may need to discuss sensitive issues with people, they do not provide us with a real office, just a glorified cubical.  So no door and walls that do not reach the ceiling.  So I need to whipser or run off to a meeting room every time I want to talk in private to an employee.  It is a pain
I also have a situation in which my sister and the woman that sits near my "office" have the same first name.
Last week I get a call from my brother and he is complaining about something with my sister and as I was on lunch I talked a bit.  This is a sister I am not fond of, none of us are.  She tests the thickness of blood to the degree that it should not be tested !
Well later in the day the woman that I work with comes in my office looking like a wounded animal.  She had a look on her face that made it look like she had just been told her puppy died.  She sits down and starts to cry.  And I look at her and ask what is wrong?  
Now before I go on, this woman is a 20-something woman that is hard to supervise as every time you tell her she did something wrong she acts as if it is a personal insult and crawls into a ball.  I have seen this, her past supervisors have and she in fact admits she is too sensitive.
Seems she heard my call and assumed all I said about my sister was about her!
Ok I spoke in hushed tones so she had to TRY to listen.
Maybe it would have been best to let it slide, because from now on I know she is an easedropper.

People need to realize that showing you are listeningto other's conversation is not a great idea.  Had a girl that was a friend, in fact for a brief time she was even my girlfriend in college.  It was so brief I hardly call it a relationship.  But we would eat out at times as her dad was some big lobbyist and would invite me and her to dinner, he liked me and was pushing me as a boyfriend.  More than once he had something come up at the last minute and would leave us to eat alone....looking back, the 5 or 6 times he did it, maybe one was a way to get us to have dinner alone, the rest were legit emergencies.
The reason I bring her up is that more than a few times we would be eating and someone at the next table would be trying to remember a movie name or a person's name or something.   I would almost have to tackle and gag this girl to keep her from answering!!!!
If I tell you that answer, it tells them yo are listening to their conversation!
And she never could get that doing that was not a great idea.  "But they want the name and all I am doing is helping"   
I never can understand why people do not get that while in this world of the cell phone some idiots will be telling their life story to a train full of people at the top of their lungs, most people like the idea of privacy and to indicate to someone that you are listening to every word they say is maybe not the best idea.
Or am I wrong?