To sit there and say one is tactically aware at all times is laughable at best. How do you do your shopping? You find the aisle with your product, do a 360 check, then walk down the aisle, doing another 360 check when you find and are facing your product, then do you grab the product, put it back down to do yet another check, then eventually place it in your grocery cart? What happens when you load your groceries? Do you open your door 1", do a 360 degree check, open the door another inch, another 360 check, open a little more, another 360 degree check, etc.. Do you make sure you _never_ use two hands when loading groceries or pushing a grocery cart? Please, OCers have just as much situation preparedness as anyone else. Situational preparedness has limitations, unless you go through life doing what 99.99% of people would consider extremely weird behavior (like walking forwards while constantly spinning 360 degrees, checking ever dumpster in the alley before walking past the alley, etc.).
Aw come on. Stop with the strawman already.
There's a HUGE difference in the situations you insist on using (shopping in a busy store) and the situation which is the topic of this thread: at 4AM on a street where there are three people: 1 victim and two robbers, and the victim didn't know there was anyone else nearby until he FELT something pressing against his head.
In your example, it's busy - there are people everywhere, many within several feet of you.
In the topic of this thread, there SHOULD be NOBODY nearby. The fact is that it was 4am, presumably quiet, probably no "hustle and bustle" to distract you, and apparently no distracting activities you must perform (driving, shopping, loading your trunk). If there is ANYONE within 200', what better thing would you have to do than to stay aware of them? If there's TWO people running up behind you, don't you think that even a marginal amount of situational awareness would suggest turning around to see what they're doing?
You make it sound like being concerned about two people behind you at 4am is an unrealistic concept.