It may be some deeply ingrained survival instinct.
It's not in the same context the OP was speaking on but I've talked to a number of ex-military people(special forces commando types) both on and offline and read a lot of literature on the subject, and they all say that humans can indeed 'feel' when someone is watching them, especially the head/face area.
For instance if you're sneaking up behind someone(i.e: a sentry) it's best to look at their legs/ass area. Apparently when you look directly at the back of their heads they invariably sense it and turn around.
Another example would be a sniper looking at someone through the scope of his rifle. Some guys believe the scope somehow amplifies the killing intent or whatever. Apparently the target starts getting anxious, starts fidgeting around etc.