Let us say you walk into a shop, find no shopkeeper inside and so you walk away. Now if you hear someone yelling a loud "hello saar" from behind, you turn around. Even if you have context-switched into a different thought stream - you do turn around instinctively. Maybe something has programmed your brain that you might be the stimulus for that call. This instinct to turn back, though quite amazing in itself, is at least acceptable. Imagine this, you are walking in a crowded street and your friend happens to see you and claps his hand. You instinctively turn back. Here there is no obvious stimulus that has programmed your brain. Furthermore - not many among the general public who hear the call turn around. In fact sometimes as a casual observer one can see only the intended recipient turn back and respond to that clap. Yes, this theory is certainly not perfect - but in the instances where it is true - it is quite fascinating.
I have heard theories floated around that in a crowded place if you want to grab the attention of someone seated at a distance - all you have to do is stare at the person intensely. Apparently, something stirs within that person and causes them to look around for potential stare'ers. Women often claim to have this magical skill of being able to know if someone is ogling at them. There is a good chance that this might be true. Many men will readily agree to an experience of having a girl turn back and give them a quick look if you have been surreptitiously sight adichufying them. I don't know what causes this so called 'sixth sense'. What stirs us from our reverie and causes us to look around? Since we now are researching everything under the sun and developing theories on them, I am sure there must be research behind this.
Having developed this "sure shot" theory, I was mildly surprised when a relative of mine complained about my responsiveness yesterday. They had seen me walking on the road and called my name. Several times. Apparently I hadn't responded. I guess no theory is perfect.