For me hypervigilance is that awful feeling of impending crisis, I know something is wrong, it feels like it did when XYZ happens (if you can put your finger on it most of the time you are in trouble) and you start scanning everything, behavior, environment, passers-by, vehicle traffic, everything looking for what is about to happen.
Unfortunately this impacts severely on others, when people are hyper-aroused, they are expecting catastrophe, so they see basically everything as potentially dangerous or harmful. While it is usually no disassociation (we know where we are, what we are doing and we are 'in' reality and the moment), our take is skewed badly by both our experiences of traumatic incidents (and that knowledge that the world is not a safe place and/or what CAN happen) which increases situational awareness all by itself.
Our ability to see every potential cause of harm, regardless of probability. With time, it becomes possible to analyze the possibles to isolate the probables, which enables us to interact with the world more effectively (it is an art not a science). The only recovery from hypervigilance I've ever found is on this basis, only reacting to things that are at least likely to go wrong unless something is changed, rather than trying to react to everything that could possibly go wrong, it makes a major difference, but it can still be noticeable (although not as exhausting).
