Hypervigilance isn't just the startle response though (I've got a huge startle response too - I jump at my own hair touching my arm and my own shadow!)

It's also when you don't really pay attention to it, but as SOON as someone else's facial or body expressions changes... you jump immediately into high alert. Or where you notice the really subtle changes over time of how someone acts/talks around you. At first you don't know that you're noticing them, but after noticing how you've reacted you can usually figured out what caused the reaction.