Nocturnus31
If you grow up in a country that is under constant threat of homicide bombings, rocket attacks, random shootings, etc. you really do begin to develop this sense. Your tactical awareness is ALWAYS on high. It's "see something, say something" on steroids.
Having worked in hot spots like Laos, Afghanistan, Pakistan, Iran, Oman, I can attest to this sense. The fact that I am here responding show that it probably works.
Most people here have grown up in this insulated America where other than the occasional maniacal shooting spree, there is not much danger of a confrontation, (outside of inner city areas). In most places outside of N. America and western Europe, this not the case. In most of the world, there is the real danger of an armed confrontation. I bet that the residents of places like Tijuana have a good idea of when it's safe to be out and about, and when it's time to take cover before the shootings starts.
Talk to anyone who has had these experiences and they will probably agree.