The rhythm and flow of people and things around you is something you pick up on, usually subconsciously, as stated in the Marine Corp Combat Hunter Program, like this.
The one I use the most is throwing a pebble into a still pond. As the ripples expand in almost perfect circles, little things under the surface cause slight changes in the ripples, or if the wind catches the tip of a ripple it will change slightly.
Like the note played wrong some how, the ripple has changed, something is not in the rhythm and flow of the world around us. We do not see the object just under the water, but we see the effects in the flow or the circle moving across the still pond.
Danger is often just like that we do not see the bomb, or the shooter waiting, but we do see, perhaps just in our subconscious the rhythm and flow of the world passing by has changed. We need to listen to that rhythm and flow around us all the time. When a person passing moves off line slightly, do they notice something we do not see. When that car passing slows down almost imperceptible, are they trying to set up a shot. You may not see the car behind you slow down, but the engine noise is just the slightest bit different, as the window goes down in the car, the car radio noise is different, the wind noise bouncing off the car window changes, you may even feel the wind change as the car slows and the window comes down.
We mark it off to many things, Scientific American reported on a study of our 6th sense as many call it. They make the argument that it is all of our senses taking in information, usually processing it in our subconscious, which then gives us a little warning bell, sometimes even a big shout that something in the flow of the world is not in sync or rhythm. Learn to listen to it, notice it, and perhaps even react a bit to it.
As the new Star Wars Movie is upon us, let me leave you with this, I feel a change in the force, perhaps there is more to it than just words.