People Watching

I am a people watcher. I love going to a public places and pretend to read a book while I actually watched others interact with each other. I'd go to the park and watch kids play, and their parents try to interact with each other. My most favorite was to grab a meal out on a patio next to a busy walking area. Before you write me off as a creeper, I actually do this for a purpose. My reasoning is I love learning about people and you can learn so much through someone else's behavior that is mainly subconscious.
For instance I caught myself looking at a new couple. All the signs where there, the puppy dog eyes, the dumb smile, the giggly laughter etc. Then I happened to look over again about ten minutes later.... It was obvious that the boy had said something that he shouldn't have, my first thought, that didn't take long. The dynamics of the entire instance had changed. While I was unsure of what said you can see she was pissed. Now how is it that me, an observer sitting 15 feet away out of earshot, could pick up on all this. Likewise many people don't realize that an observer like me, can still pick up on what just happened even though I have no idea what was said. Well boys and girls, it's called nonverbal communication.
Any gesture, motion, hair flick, batting of eyes, the way your arms are position, eye contact, facial expression, etc. everything you do sends a message to the receiving party. Many refer to it as a 'feeling' they had, or they just 'knew'. That really means they picked up on the non verbals of the conversation and they can't place them into words.
Now I pose the question, me watching this couple from a distance and watching for their non verbals, was that an invasion of privacy? Granted I'm not stalking them, didn't take pictures or anything and after all it was in public. I know we have all people watched at one point in our life, but where is that line between innocent watching and invasion of privacy?
Listening in on phone calls, watching through windows and following people is obviously invasion of privacy. Taking pictures, stealing personal items and such all fall under this category but what about watching the non verbals in someone else's argument? Me sitting there and following a dialog through non verbals, was that crossing a line or would it fall under being nosy?
After all it was all out and the open, that makes me wonder if they even cared. I'm sure many don't realize that someone can follow their conversation without actually being in it. I figure in this situation I'm going to rely on what my Grandma told me awhile ago. If you don't want everyone to know, don't do it in public.
I still people watch even though I consider watching to much being an invasion of privacy from time to time.