I think most peolpe would be surprised at the ability the brain has to analyse stimulus that you may not be conciously be aware of. The brain filters out a *huge* amount of information (auditory and visual) so what you actually "see"/"hear" isn't the limit of what your senses are picking up, it is simply what's left after pre-processing. (nb. I'm NOT going towards esp etc.) This can result in feelings like deja Vu (this is where the brain has reprocessed an image after storing the first run through giving you the sense you've seen it before) or other "strange" feelings related to sense. This discarding of information also neatly explains the more adept olfactory and auditory senses of the blind, as their brain will discard less, rather than the sense actaully becoming stronger, they are just analysing the data in more detail and not scrapping as much. As far as this related to telling distance and location of people behind you, this is really no different to the sort of instinct reaction to catch somethingt thrown at you. Although spinning ballistic trajectories are mathematically relatively complicated (and would be impossible to conciously solve before the object reached you) the subconcious is able to do a very fast solution and project of motion and coordinate your hands to catch, even accounting for the angular motion of the incoming object. I know this has been a bit rambling, but I'm just trying to say that strange feelings tend to be when the data usually scrapped by the subconcious is either double processed or suddenly deemed important to make conciously available (i.e. appealing to the higher brain functions to make a decision of action based on the stimulus).