It's not PSTD as others are saying, it's Se, I relate to this completely.
I also take notice of chairs, bottles, utensils, anything and everything can be used as a weapon. I pay attention to the direction people are facing too, to see if I can surprise them or throw them off balance if need be. Any posts or corners or desks which can be used to shield against projectiles. I do feel very in-tune with my environment, and am constantly watching how things develop, changes in my environment.
As far as the facing potential threats, I practically *never* turn my back to anyone in my environment. Not unless I'm already walking or moving, and can easily hear/peripherally see an oncoming attacker from behind. In this regard, I always err on the side of being socially awkward, usually joking somehow to excuse it, and being safe.
In restaurants, always facing the entrance, or windows, to see people entering. Always scanning for exits, should there be an emergency. Thinking about what objects I could use to throw in certain directions or angles if I need to cause a distraction, or attack somebody who's threatening me and/or the rest of the group.
... I can understand how this may sound stressful to those who don't do it, but it's just something that comes very naturally. And very quickly when you do it all the time.